Unexpected var, use let or const instead. Open
var timeObject = function(timeType, value) {
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require let
or const
instead of var
(no-var)
ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let
and const
keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes
such as:
var count = people.length;
var enoughFood = count > sandwiches.length;
if (enoughFood) {
var count = sandwiches.length; // accidentally overriding the count variable
console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}
// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");
Rule Details
This rule is aimed at discouraging the use of var
and encouraging the use of const
or let
instead.
Examples
Examples of incorrect code for this rule:
/*eslint no-var: "error"*/
var x = "y";
var CONFIG = {};
Examples of correct code for this rule:
/*eslint no-var: "error"*/
/*eslint-env es6*/
let x = "y";
const CONFIG = {};
When Not To Use It
In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their
codebase may not want to apply this rule if the cost of migrating from var
to let
is too costly.
Source: http://eslint.org/docs/rules/
A space is required before '}'. Open
return {text: value + ' ' + timeType, value: value};
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enforce consistent spacing inside braces (object-curly-spacing)
While formatting preferences are very personal, a number of style guides require or disallow spaces between curly braces in the following situations:
// simple object literals
var obj = { foo: "bar" };
// nested object literals
var obj = { foo: { zoo: "bar" } };
// destructuring assignment (EcmaScript 6)
var { x, y } = y;
// import/export declarations (EcmaScript 6)
import { foo } from "bar";
export { foo };
Rule Details
This rule enforces consistent spacing inside braces of object literals, destructuring assignments, and import/export specifiers.
Options
This rule has two options, a string option and an object option.
String option:
-
"never"
(default) disallows spacing inside of braces -
"always"
requires spacing inside of braces (except{}
)
Object option:
-
"arraysInObjects": true
requires spacing inside of braces of objects beginning and/or ending with an array element (applies when the first option is set tonever
) -
"arraysInObjects": false
disallows spacing inside of braces of objects beginning and/or ending with an array element (applies when the first option is set toalways
) -
"objectsInObjects": true
requires spacing inside of braces of objects beginning and/or ending with an object element (applies when the first option is set tonever
) -
"objectsInObjects": false
disallows spacing inside of braces of objects beginning and/or ending with an object element (applies when the first option is set toalways
)
never
Examples of incorrect code for this rule with the default "never"
option:
/*eslint object-curly-spacing: ["error", "never"]*/
var obj = { 'foo': 'bar' };
var obj = {'foo': 'bar' };
var obj = { baz: {'foo': 'qux'}, bar};
var obj = {baz: { 'foo': 'qux'}, bar};
var {x } = y;
import { foo } from 'bar';
Examples of correct code for this rule with the default "never"
option:
/*eslint object-curly-spacing: ["error", "never"]*/
var obj = {'foo': 'bar'};
var obj = {'foo': {'bar': 'baz'}, 'qux': 'quxx'};
var obj = {
'foo': 'bar'
};
var obj = {'foo': 'bar'
};
var obj = {
'foo':'bar'};
var obj = {};
var {x} = y;
import {foo} from 'bar';
always
Examples of incorrect code for this rule with the "always"
option:
/*eslint object-curly-spacing: ["error", "always"]*/
var obj = {'foo': 'bar'};
var obj = {'foo': 'bar' };
var obj = { baz: {'foo': 'qux'}, bar};
var obj = {baz: { 'foo': 'qux' }, bar};
var obj = {'foo': 'bar'
};
var obj = {
'foo':'bar'};
var {x} = y;
import {foo } from 'bar';
Examples of correct code for this rule with the "always"
option:
/*eslint object-curly-spacing: ["error", "always"]*/
var obj = {};
var obj = { 'foo': 'bar' };
var obj = { 'foo': { 'bar': 'baz' }, 'qux': 'quxx' };
var obj = {
'foo': 'bar'
};
var { x } = y;
import { foo } from 'bar';
arraysInObjects
Examples of additional correct code for this rule with the "never", { "arraysInObjects": true }
options:
/*eslint object-curly-spacing: ["error", "never", { "arraysInObjects": true }]*/
var obj = {"foo": [ 1, 2 ] };
var obj = {"foo": [ "baz", "bar" ] };
Examples of additional correct code for this rule with the "always", { "arraysInObjects": false }
options:
/*eslint object-curly-spacing: ["error", "always", { "arraysInObjects": false }]*/
var obj = { "foo": [ 1, 2 ]};
var obj = { "foo": [ "baz", "bar" ]};
objectsInObjects
Examples of additional correct code for this rule with the "never", { "objectsInObjects": true }
options:
/*eslint object-curly-spacing: ["error", "never", { "objectsInObjects": true }]*/
var obj = {"foo": {"baz": 1, "bar": 2} };
Examples of additional correct code for this rule with the "always", { "objectsInObjects": false }
options:
/*eslint object-curly-spacing: ["error", "always", { "objectsInObjects": false }]*/
var obj = { "foo": { "baz": 1, "bar": 2 }};
When Not To Use It
You can turn this rule off if you are not concerned with the consistency of spacing between curly braces.
Related Rules
- [array-bracket-spacing](array-bracket-spacing.md)
- [comma-spacing](comma-spacing.md)
- [computed-property-spacing](computed-property-spacing.md)
- [space-in-parens](space-in-parens.md) Source: http://eslint.org/docs/rules/
Unexpected var, use let or const instead. Open
var hourlyTimeOptions = _.flatten([
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require let
or const
instead of var
(no-var)
ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let
and const
keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes
such as:
var count = people.length;
var enoughFood = count > sandwiches.length;
if (enoughFood) {
var count = sandwiches.length; // accidentally overriding the count variable
console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}
// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");
Rule Details
This rule is aimed at discouraging the use of var
and encouraging the use of const
or let
instead.
Examples
Examples of incorrect code for this rule:
/*eslint no-var: "error"*/
var x = "y";
var CONFIG = {};
Examples of correct code for this rule:
/*eslint no-var: "error"*/
/*eslint-env es6*/
let x = "y";
const CONFIG = {};
When Not To Use It
In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their
codebase may not want to apply this rule if the cost of migrating from var
to let
is too costly.
Source: http://eslint.org/docs/rules/
Unnecessarily quoted property 'Daily' found. Open
'Daily': timeDataBuilder('Days', 6),
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require quotes around object literal property names (quote-props)
Object literal property names can be defined in two ways: using literals or using strings. For example, these two objects are equivalent:
var object1 = {
property: true
};
var object2 = {
"property": true
};
In many cases, it doesn't matter if you choose to use an identifier instead of a string or vice-versa. Even so, you might decide to enforce a consistent style in your code.
There are, however, some occasions when you must use quotes:
- If you are using an ECMAScript 3 JavaScript engine (such as IE8) and you want to use a keyword (such as
if
) as a property name. This restriction was removed in ECMAScript 5. - You want to use a non-identifier character in your property name, such as having a property with a space like
"one two"
.
Another example where quotes do matter is when using numeric literals as property keys:
var object = {
1e2: 1,
100: 2
};
This may look alright at first sight, but this code in fact throws a syntax error in ECMAScript 5 strict mode. This happens because 1e2
and 100
are coerced into strings before getting used as the property name. Both String(1e2)
and String(100)
happen to be equal to "100"
, which causes the "Duplicate data property in object literal not allowed in strict mode" error. Issues like that can be tricky to debug, so some prefer to require quotes around all property names.
Rule Details
This rule requires quotes around object literal property names.
Options
This rule has two options, a string option and an object option.
String option:
-
"always"
(default) requires quotes around all object literal property names -
"as-needed"
disallows quotes around object literal property names that are not strictly required -
"consistent"
enforces a consistent quote style; in a given object, either all of the properties should be quoted, or none of the properties should be quoted -
"consistent-as-needed"
requires quotes around all object literal property names if any name strictly requires quotes, otherwise disallows quotes around object property names
Object option:
-
"keywords": true
requires quotes around language keywords used as object property names (only applies when usingas-needed
orconsistent-as-needed
) -
"unnecessary": true
(default) disallows quotes around object literal property names that are not strictly required (only applies when usingas-needed
) -
"unnecessary": false
allows quotes around object literal property names that are not strictly required (only applies when usingas-needed
) -
"numbers": true
requires quotes around numbers used as object property names (only applies when usingas-needed
)
always
Examples of incorrect code for this rule with the default "always"
option:
/*eslint quote-props: ["error", "always"]*/
var object = {
foo: "bar",
baz: 42,
"qux-lorem": true
};
Examples of correct code for this rule with the default "always"
option:
/*eslint quote-props: ["error", "always"]*/
/*eslint-env es6*/
var object1 = {
"foo": "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
'foo': 'bar',
'baz': 42,
'qux-lorem': true
};
var object3 = {
foo() {
return;
}
};
as-needed
Examples of incorrect code for this rule with the "as-needed"
option:
/*eslint quote-props: ["error", "as-needed"]*/
var object = {
"a": 0,
"0": 0,
"true": 0,
"null": 0
};
Examples of correct code for this rule with the "as-needed"
option:
/*eslint quote-props: ["error", "as-needed"]*/
/*eslint-env es6*/
var object1 = {
"a-b": 0,
"0x0": 0,
"1e2": 0
};
var object2 = {
foo: 'bar',
baz: 42,
true: 0,
0: 0,
'qux-lorem': true
};
var object3 = {
foo() {
return;
}
};
consistent
Examples of incorrect code for this rule with the "consistent"
option:
/*eslint quote-props: ["error", "consistent"]*/
var object1 = {
foo: "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
'foo': 'bar',
baz: 42
};
Examples of correct code for this rule with the "consistent"
option:
/*eslint quote-props: ["error", "consistent"]*/
var object1 = {
"foo": "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
'foo': 'bar',
'baz': 42
};
var object3 = {
foo: 'bar',
baz: 42
};
consistent-as-needed
Examples of incorrect code for this rule with the "consistent-as-needed"
option:
/*eslint quote-props: ["error", "consistent-as-needed"]*/
var object1 = {
foo: "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
'foo': 'bar',
'baz': 42
};
Examples of correct code for this rule with the "consistent-as-needed"
option:
/*eslint quote-props: ["error", "consistent-as-needed"]*/
var object1 = {
"foo": "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
foo: 'bar',
baz: 42
};
keywords
Examples of additional incorrect code for this rule with the "as-needed", { "keywords": true }
options:
/*eslint quote-props: ["error", "as-needed", { "keywords": true }]*/
var x = {
while: 1,
volatile: "foo"
};
Examples of additional incorrect code for this rule with the "consistent-as-needed", { "keywords": true }
options:
/*eslint quote-props: ["error", "consistent-as-needed", { "keywords": true }]*/
var x = {
"prop": 1,
"bar": "foo"
};
unnecessary
Examples of additional correct code for this rule with the "as-needed", { "unnecessary": false }
options:
/*eslint quote-props: ["error", "as-needed", { "keywords": true, "unnecessary": false }]*/
var x = {
"while": 1,
"foo": "bar" // Would normally have caused a warning
};
numbers
Examples of additional incorrect code for this rule with the "as-needed", { "numbers": true }
options:
/*eslint quote-props: ["error", "as-needed", { "numbers": true }]*/
var x = {
100: 1
}
When Not To Use It
If you don't care if property names are consistently wrapped in quotes or not, and you don't target legacy ES3 environments, turn this rule off.
Further Reading
Expected property shorthand. Open
return {text: value + ' ' + timeType, value: value};
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Require Object Literal Shorthand Syntax (object-shorthand)
ECMAScript 6 provides a concise form for defining object literal methods and properties. This syntax can make defining complex object literals much cleaner.
Here are a few common examples using the ES5 syntax:
// properties
var foo = {
x: x,
y: y,
z: z,
};
// methods
var foo = {
a: function() {},
b: function() {}
};
Now here are ES6 equivalents:
/*eslint-env es6*/
// properties
var foo = {x, y, z};
// methods
var foo = {
a() {},
b() {}
};
Rule Details
This rule enforces the use of the shorthand syntax. This applies to all methods (including generators) defined in object literals and any properties defined where the key name matches name of the assigned variable.
Each of the following properties would warn:
/*eslint object-shorthand: "error"*/
/*eslint-env es6*/
var foo = {
w: function() {},
x: function *() {},
[y]: function() {},
z: z
};
In that case the expected syntax would have been:
/*eslint object-shorthand: "error"*/
/*eslint-env es6*/
var foo = {
w() {},
*x() {},
[y]() {},
z
};
This rule does not flag arrow functions inside of object literals. The following will not warn:
/*eslint object-shorthand: "error"*/
/*eslint-env es6*/
var foo = {
x: (y) => y
};
See Also:
-
no-useless-rename
which disallows renaming import, export, and destructured assignments to the same name.
Options
The rule takes an option which specifies when it should be applied. It can be set to one of the following values:
-
"always"
(default) expects that the shorthand will be used whenever possible. -
"methods"
ensures the method shorthand is used (also applies to generators). -
"properties"
ensures the property shorthand is used (where the key and variable name match). -
"never"
ensures that no property or method shorthand is used in any object literal. -
"consistent"
ensures that either all shorthand or all long-form will be used in an object literal. -
"consistent-as-needed"
ensures that either all shorthand or all long-form will be used in an object literal, but ensures all shorthand whenever possible.
You can set the option in configuration like this:
{
"object-shorthand": ["error", "always"]
}
Additionally, the rule takes an optional object configuration:
-
"avoidQuotes": true
indicates that long-form syntax is preferred whenever the object key is a string literal (default:false
). Note that this option can only be enabled when the string option is set to"always"
,"methods"
, or"properties"
. -
"ignoreConstructors": true
can be used to prevent the rule from reporting errors for constructor functions. (By default, the rule treats constructors the same way as other functions.) Note that this option can only be enabled when the string option is set to"always"
or"methods"
. -
"avoidExplicitReturnArrows": true
indicates that methods are preferred over explicit-return arrow functions for function properties. (By default, the rule allows either of these.) Note that this option can only be enabled when the string option is set to"always"
or"methods"
.
avoidQuotes
{
"object-shorthand": ["error", "always", { "avoidQuotes": true }]
}
Example of incorrect code for this rule with the "always", { "avoidQuotes": true }
option:
/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/
var foo = {
"bar-baz"() {}
};
Example of correct code for this rule with the "always", { "avoidQuotes": true }
option:
/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/
var foo = {
"bar-baz": function() {},
"qux": qux
};
ignoreConstructors
{
"object-shorthand": ["error", "always", { "ignoreConstructors": true }]
}
Example of correct code for this rule with the "always", { "ignoreConstructors": true }
option:
/*eslint object-shorthand: ["error", "always", { "ignoreConstructors": true }]*/
/*eslint-env es6*/
var foo = {
ConstructorFunction: function() {}
};
avoidExplicitReturnArrows
{
"object-shorthand": ["error", "always", { "avoidExplicitReturnArrows": true }]
}
Example of incorrect code for this rule with the "always", { "avoidExplicitReturnArrows": true }
option:
/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/
var foo = {
foo: (bar, baz) => {
return bar + baz;
},
qux: (foobar) => {
return foobar * 2;
}
};
Example of correct code for this rule with the "always", { "avoidExplicitReturnArrows": true }
option:
/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/
var foo = {
foo(bar, baz) {
return bar + baz;
},
qux: foobar => foobar * 2
};
Example of incorrect code for this rule with the "consistent"
option:
/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/
var foo = {
a,
b: "foo",
};
Examples of correct code for this rule with the "consistent"
option:
/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/
var foo = {
a: a,
b: "foo"
};
var bar = {
a,
b,
};
Example of incorrect code with the "consistent-as-needed"
option, which is very similar to "consistent"
:
/*eslint object-shorthand: [2, "consistent-as-needed"]*/
/*eslint-env es6*/
var foo = {
a: a,
b: b,
};
When Not To Use It
Anyone not yet in an ES6 environment would not want to apply this rule. Others may find the terseness of the shorthand syntax harder to read and may not want to encourage it with this rule.
Further Reading
Object initializer - MDN Source: http://eslint.org/docs/rules/
Unnecessarily quoted property 'Weekly' found. Open
'Weekly': timeDataBuilder('Weeks', 4),
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require quotes around object literal property names (quote-props)
Object literal property names can be defined in two ways: using literals or using strings. For example, these two objects are equivalent:
var object1 = {
property: true
};
var object2 = {
"property": true
};
In many cases, it doesn't matter if you choose to use an identifier instead of a string or vice-versa. Even so, you might decide to enforce a consistent style in your code.
There are, however, some occasions when you must use quotes:
- If you are using an ECMAScript 3 JavaScript engine (such as IE8) and you want to use a keyword (such as
if
) as a property name. This restriction was removed in ECMAScript 5. - You want to use a non-identifier character in your property name, such as having a property with a space like
"one two"
.
Another example where quotes do matter is when using numeric literals as property keys:
var object = {
1e2: 1,
100: 2
};
This may look alright at first sight, but this code in fact throws a syntax error in ECMAScript 5 strict mode. This happens because 1e2
and 100
are coerced into strings before getting used as the property name. Both String(1e2)
and String(100)
happen to be equal to "100"
, which causes the "Duplicate data property in object literal not allowed in strict mode" error. Issues like that can be tricky to debug, so some prefer to require quotes around all property names.
Rule Details
This rule requires quotes around object literal property names.
Options
This rule has two options, a string option and an object option.
String option:
-
"always"
(default) requires quotes around all object literal property names -
"as-needed"
disallows quotes around object literal property names that are not strictly required -
"consistent"
enforces a consistent quote style; in a given object, either all of the properties should be quoted, or none of the properties should be quoted -
"consistent-as-needed"
requires quotes around all object literal property names if any name strictly requires quotes, otherwise disallows quotes around object property names
Object option:
-
"keywords": true
requires quotes around language keywords used as object property names (only applies when usingas-needed
orconsistent-as-needed
) -
"unnecessary": true
(default) disallows quotes around object literal property names that are not strictly required (only applies when usingas-needed
) -
"unnecessary": false
allows quotes around object literal property names that are not strictly required (only applies when usingas-needed
) -
"numbers": true
requires quotes around numbers used as object property names (only applies when usingas-needed
)
always
Examples of incorrect code for this rule with the default "always"
option:
/*eslint quote-props: ["error", "always"]*/
var object = {
foo: "bar",
baz: 42,
"qux-lorem": true
};
Examples of correct code for this rule with the default "always"
option:
/*eslint quote-props: ["error", "always"]*/
/*eslint-env es6*/
var object1 = {
"foo": "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
'foo': 'bar',
'baz': 42,
'qux-lorem': true
};
var object3 = {
foo() {
return;
}
};
as-needed
Examples of incorrect code for this rule with the "as-needed"
option:
/*eslint quote-props: ["error", "as-needed"]*/
var object = {
"a": 0,
"0": 0,
"true": 0,
"null": 0
};
Examples of correct code for this rule with the "as-needed"
option:
/*eslint quote-props: ["error", "as-needed"]*/
/*eslint-env es6*/
var object1 = {
"a-b": 0,
"0x0": 0,
"1e2": 0
};
var object2 = {
foo: 'bar',
baz: 42,
true: 0,
0: 0,
'qux-lorem': true
};
var object3 = {
foo() {
return;
}
};
consistent
Examples of incorrect code for this rule with the "consistent"
option:
/*eslint quote-props: ["error", "consistent"]*/
var object1 = {
foo: "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
'foo': 'bar',
baz: 42
};
Examples of correct code for this rule with the "consistent"
option:
/*eslint quote-props: ["error", "consistent"]*/
var object1 = {
"foo": "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
'foo': 'bar',
'baz': 42
};
var object3 = {
foo: 'bar',
baz: 42
};
consistent-as-needed
Examples of incorrect code for this rule with the "consistent-as-needed"
option:
/*eslint quote-props: ["error", "consistent-as-needed"]*/
var object1 = {
foo: "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
'foo': 'bar',
'baz': 42
};
Examples of correct code for this rule with the "consistent-as-needed"
option:
/*eslint quote-props: ["error", "consistent-as-needed"]*/
var object1 = {
"foo": "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
foo: 'bar',
baz: 42
};
keywords
Examples of additional incorrect code for this rule with the "as-needed", { "keywords": true }
options:
/*eslint quote-props: ["error", "as-needed", { "keywords": true }]*/
var x = {
while: 1,
volatile: "foo"
};
Examples of additional incorrect code for this rule with the "consistent-as-needed", { "keywords": true }
options:
/*eslint quote-props: ["error", "consistent-as-needed", { "keywords": true }]*/
var x = {
"prop": 1,
"bar": "foo"
};
unnecessary
Examples of additional correct code for this rule with the "as-needed", { "unnecessary": false }
options:
/*eslint quote-props: ["error", "as-needed", { "keywords": true, "unnecessary": false }]*/
var x = {
"while": 1,
"foo": "bar" // Would normally have caused a warning
};
numbers
Examples of additional incorrect code for this rule with the "as-needed", { "numbers": true }
options:
/*eslint quote-props: ["error", "as-needed", { "numbers": true }]*/
var x = {
100: 1
}
When Not To Use It
If you don't care if property names are consistently wrapped in quotes or not, and you don't target legacy ES3 environments, turn this rule off.
Further Reading
A space is required after '{'. Open
return {text: value + ' ' + timeType, value: value};
- Read upRead up
- Create a ticketCreate a ticket
- Exclude checks
enforce consistent spacing inside braces (object-curly-spacing)
While formatting preferences are very personal, a number of style guides require or disallow spaces between curly braces in the following situations:
// simple object literals
var obj = { foo: "bar" };
// nested object literals
var obj = { foo: { zoo: "bar" } };
// destructuring assignment (EcmaScript 6)
var { x, y } = y;
// import/export declarations (EcmaScript 6)
import { foo } from "bar";
export { foo };
Rule Details
This rule enforces consistent spacing inside braces of object literals, destructuring assignments, and import/export specifiers.
Options
This rule has two options, a string option and an object option.
String option:
-
"never"
(default) disallows spacing inside of braces -
"always"
requires spacing inside of braces (except{}
)
Object option:
-
"arraysInObjects": true
requires spacing inside of braces of objects beginning and/or ending with an array element (applies when the first option is set tonever
) -
"arraysInObjects": false
disallows spacing inside of braces of objects beginning and/or ending with an array element (applies when the first option is set toalways
) -
"objectsInObjects": true
requires spacing inside of braces of objects beginning and/or ending with an object element (applies when the first option is set tonever
) -
"objectsInObjects": false
disallows spacing inside of braces of objects beginning and/or ending with an object element (applies when the first option is set toalways
)
never
Examples of incorrect code for this rule with the default "never"
option:
/*eslint object-curly-spacing: ["error", "never"]*/
var obj = { 'foo': 'bar' };
var obj = {'foo': 'bar' };
var obj = { baz: {'foo': 'qux'}, bar};
var obj = {baz: { 'foo': 'qux'}, bar};
var {x } = y;
import { foo } from 'bar';
Examples of correct code for this rule with the default "never"
option:
/*eslint object-curly-spacing: ["error", "never"]*/
var obj = {'foo': 'bar'};
var obj = {'foo': {'bar': 'baz'}, 'qux': 'quxx'};
var obj = {
'foo': 'bar'
};
var obj = {'foo': 'bar'
};
var obj = {
'foo':'bar'};
var obj = {};
var {x} = y;
import {foo} from 'bar';
always
Examples of incorrect code for this rule with the "always"
option:
/*eslint object-curly-spacing: ["error", "always"]*/
var obj = {'foo': 'bar'};
var obj = {'foo': 'bar' };
var obj = { baz: {'foo': 'qux'}, bar};
var obj = {baz: { 'foo': 'qux' }, bar};
var obj = {'foo': 'bar'
};
var obj = {
'foo':'bar'};
var {x} = y;
import {foo } from 'bar';
Examples of correct code for this rule with the "always"
option:
/*eslint object-curly-spacing: ["error", "always"]*/
var obj = {};
var obj = { 'foo': 'bar' };
var obj = { 'foo': { 'bar': 'baz' }, 'qux': 'quxx' };
var obj = {
'foo': 'bar'
};
var { x } = y;
import { foo } from 'bar';
arraysInObjects
Examples of additional correct code for this rule with the "never", { "arraysInObjects": true }
options:
/*eslint object-curly-spacing: ["error", "never", { "arraysInObjects": true }]*/
var obj = {"foo": [ 1, 2 ] };
var obj = {"foo": [ "baz", "bar" ] };
Examples of additional correct code for this rule with the "always", { "arraysInObjects": false }
options:
/*eslint object-curly-spacing: ["error", "always", { "arraysInObjects": false }]*/
var obj = { "foo": [ 1, 2 ]};
var obj = { "foo": [ "baz", "bar" ]};
objectsInObjects
Examples of additional correct code for this rule with the "never", { "objectsInObjects": true }
options:
/*eslint object-curly-spacing: ["error", "never", { "objectsInObjects": true }]*/
var obj = {"foo": {"baz": 1, "bar": 2} };
Examples of additional correct code for this rule with the "always", { "objectsInObjects": false }
options:
/*eslint object-curly-spacing: ["error", "always", { "objectsInObjects": false }]*/
var obj = { "foo": { "baz": 1, "bar": 2 }};
When Not To Use It
You can turn this rule off if you are not concerned with the consistency of spacing between curly braces.
Related Rules
- [array-bracket-spacing](array-bracket-spacing.md)
- [comma-spacing](comma-spacing.md)
- [computed-property-spacing](computed-property-spacing.md)
- [space-in-parens](space-in-parens.md) Source: http://eslint.org/docs/rules/
Unexpected string concatenation. Open
return {text: value + ' ' + timeType, value: value};
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Suggest using template literals instead of string concatenation. (prefer-template)
In ES2015 (ES6), we can use template literals instead of string concatenation.
var str = "Hello, " + name + "!";
/*eslint-env es6*/
var str = `Hello, ${name}!`;
Rule Details
This rule is aimed to flag usage of +
operators with strings.
Examples
Examples of incorrect code for this rule:
/*eslint prefer-template: "error"*/
var str = "Hello, " + name + "!";
var str = "Time: " + (12 * 60 * 60 * 1000);
Examples of correct code for this rule:
/*eslint prefer-template: "error"*/
/*eslint-env es6*/
var str = "Hello World!";
var str = `Hello, ${name}!`;
var str = `Time: ${12 * 60 * 60 * 1000}`;
// This is reported by `no-useless-concat`.
var str = "Hello, " + "World!";
When Not To Use It
This rule should not be used in ES3/5 environments.
In ES2015 (ES6) or later, if you don't want to be notified about string concatenation, you can safely disable this rule.
Related Rules
- [no-useless-concat](no-useless-concat.md)
- [quotes](quotes.md) Source: http://eslint.org/docs/rules/
All 'var' declarations must be at the top of the function scope. Open
for (var value = 1; value <= iterations; value++) {
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Require Variable Declarations to be at the top of their scope (vars-on-top)
The vars-on-top
rule generates warnings when variable declarations are not used serially at the top of a function scope or the top of a program.
By default variable declarations are always moved (“hoisted”) invisibly to the top of their containing scope by the JavaScript interpreter.
This rule forces the programmer to represent that behavior by manually moving the variable declaration to the top of its containing scope.
Rule Details
This rule aims to keep all variable declarations in the leading series of statements. Allowing multiple declarations helps promote maintainability and is thus allowed.
Examples of incorrect code for this rule:
/*eslint vars-on-top: "error"*/
// Variable declarations in a block:
function doSomething() {
var first;
if (true) {
first = true;
}
var second;
}
// Variable declaration in for initializer:
function doSomething() {
for (var i=0; i<10; i++) {}
}
/*eslint vars-on-top: "error"*/
// Variables after other statements:
f();
var a;
Examples of correct code for this rule:
/*eslint vars-on-top: "error"*/
function doSomething() {
var first;
var second; //multiple declarations are allowed at the top
if (true) {
first = true;
}
}
function doSomething() {
var i;
for (i=0; i<10; i++) {}
}
/*eslint vars-on-top: "error"*/
var a;
f();
/*eslint vars-on-top: "error"*/
// Directives may precede variable declarations.
"use strict";
var a;
f();
// Comments can describe variables.
function doSomething() {
// this is the first var.
var first;
// this is the second var.
var second
}
Further Reading
Unnecessarily quoted property 'Monthly' found. Open
'Monthly': timeDataBuilder('Months', 6),
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require quotes around object literal property names (quote-props)
Object literal property names can be defined in two ways: using literals or using strings. For example, these two objects are equivalent:
var object1 = {
property: true
};
var object2 = {
"property": true
};
In many cases, it doesn't matter if you choose to use an identifier instead of a string or vice-versa. Even so, you might decide to enforce a consistent style in your code.
There are, however, some occasions when you must use quotes:
- If you are using an ECMAScript 3 JavaScript engine (such as IE8) and you want to use a keyword (such as
if
) as a property name. This restriction was removed in ECMAScript 5. - You want to use a non-identifier character in your property name, such as having a property with a space like
"one two"
.
Another example where quotes do matter is when using numeric literals as property keys:
var object = {
1e2: 1,
100: 2
};
This may look alright at first sight, but this code in fact throws a syntax error in ECMAScript 5 strict mode. This happens because 1e2
and 100
are coerced into strings before getting used as the property name. Both String(1e2)
and String(100)
happen to be equal to "100"
, which causes the "Duplicate data property in object literal not allowed in strict mode" error. Issues like that can be tricky to debug, so some prefer to require quotes around all property names.
Rule Details
This rule requires quotes around object literal property names.
Options
This rule has two options, a string option and an object option.
String option:
-
"always"
(default) requires quotes around all object literal property names -
"as-needed"
disallows quotes around object literal property names that are not strictly required -
"consistent"
enforces a consistent quote style; in a given object, either all of the properties should be quoted, or none of the properties should be quoted -
"consistent-as-needed"
requires quotes around all object literal property names if any name strictly requires quotes, otherwise disallows quotes around object property names
Object option:
-
"keywords": true
requires quotes around language keywords used as object property names (only applies when usingas-needed
orconsistent-as-needed
) -
"unnecessary": true
(default) disallows quotes around object literal property names that are not strictly required (only applies when usingas-needed
) -
"unnecessary": false
allows quotes around object literal property names that are not strictly required (only applies when usingas-needed
) -
"numbers": true
requires quotes around numbers used as object property names (only applies when usingas-needed
)
always
Examples of incorrect code for this rule with the default "always"
option:
/*eslint quote-props: ["error", "always"]*/
var object = {
foo: "bar",
baz: 42,
"qux-lorem": true
};
Examples of correct code for this rule with the default "always"
option:
/*eslint quote-props: ["error", "always"]*/
/*eslint-env es6*/
var object1 = {
"foo": "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
'foo': 'bar',
'baz': 42,
'qux-lorem': true
};
var object3 = {
foo() {
return;
}
};
as-needed
Examples of incorrect code for this rule with the "as-needed"
option:
/*eslint quote-props: ["error", "as-needed"]*/
var object = {
"a": 0,
"0": 0,
"true": 0,
"null": 0
};
Examples of correct code for this rule with the "as-needed"
option:
/*eslint quote-props: ["error", "as-needed"]*/
/*eslint-env es6*/
var object1 = {
"a-b": 0,
"0x0": 0,
"1e2": 0
};
var object2 = {
foo: 'bar',
baz: 42,
true: 0,
0: 0,
'qux-lorem': true
};
var object3 = {
foo() {
return;
}
};
consistent
Examples of incorrect code for this rule with the "consistent"
option:
/*eslint quote-props: ["error", "consistent"]*/
var object1 = {
foo: "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
'foo': 'bar',
baz: 42
};
Examples of correct code for this rule with the "consistent"
option:
/*eslint quote-props: ["error", "consistent"]*/
var object1 = {
"foo": "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
'foo': 'bar',
'baz': 42
};
var object3 = {
foo: 'bar',
baz: 42
};
consistent-as-needed
Examples of incorrect code for this rule with the "consistent-as-needed"
option:
/*eslint quote-props: ["error", "consistent-as-needed"]*/
var object1 = {
foo: "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
'foo': 'bar',
'baz': 42
};
Examples of correct code for this rule with the "consistent-as-needed"
option:
/*eslint quote-props: ["error", "consistent-as-needed"]*/
var object1 = {
"foo": "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
foo: 'bar',
baz: 42
};
keywords
Examples of additional incorrect code for this rule with the "as-needed", { "keywords": true }
options:
/*eslint quote-props: ["error", "as-needed", { "keywords": true }]*/
var x = {
while: 1,
volatile: "foo"
};
Examples of additional incorrect code for this rule with the "consistent-as-needed", { "keywords": true }
options:
/*eslint quote-props: ["error", "consistent-as-needed", { "keywords": true }]*/
var x = {
"prop": 1,
"bar": "foo"
};
unnecessary
Examples of additional correct code for this rule with the "as-needed", { "unnecessary": false }
options:
/*eslint quote-props: ["error", "as-needed", { "keywords": true, "unnecessary": false }]*/
var x = {
"while": 1,
"foo": "bar" // Would normally have caused a warning
};
numbers
Examples of additional incorrect code for this rule with the "as-needed", { "numbers": true }
options:
/*eslint quote-props: ["error", "as-needed", { "numbers": true }]*/
var x = {
100: 1
}
When Not To Use It
If you don't care if property names are consistently wrapped in quotes or not, and you don't target legacy ES3 environments, turn this rule off.
Further Reading
Unexpected var, use let or const instead. Open
var singularize = function(timeType) {
- Read upRead up
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require let
or const
instead of var
(no-var)
ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let
and const
keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes
such as:
var count = people.length;
var enoughFood = count > sandwiches.length;
if (enoughFood) {
var count = sandwiches.length; // accidentally overriding the count variable
console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}
// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");
Rule Details
This rule is aimed at discouraging the use of var
and encouraging the use of const
or let
instead.
Examples
Examples of incorrect code for this rule:
/*eslint no-var: "error"*/
var x = "y";
var CONFIG = {};
Examples of correct code for this rule:
/*eslint no-var: "error"*/
/*eslint-env es6*/
let x = "y";
const CONFIG = {};
When Not To Use It
In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their
codebase may not want to apply this rule if the cost of migrating from var
to let
is too costly.
Source: http://eslint.org/docs/rules/
Expected property shorthand. Open
timeData.push({ text: singularize(timeType), value: value });
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Require Object Literal Shorthand Syntax (object-shorthand)
ECMAScript 6 provides a concise form for defining object literal methods and properties. This syntax can make defining complex object literals much cleaner.
Here are a few common examples using the ES5 syntax:
// properties
var foo = {
x: x,
y: y,
z: z,
};
// methods
var foo = {
a: function() {},
b: function() {}
};
Now here are ES6 equivalents:
/*eslint-env es6*/
// properties
var foo = {x, y, z};
// methods
var foo = {
a() {},
b() {}
};
Rule Details
This rule enforces the use of the shorthand syntax. This applies to all methods (including generators) defined in object literals and any properties defined where the key name matches name of the assigned variable.
Each of the following properties would warn:
/*eslint object-shorthand: "error"*/
/*eslint-env es6*/
var foo = {
w: function() {},
x: function *() {},
[y]: function() {},
z: z
};
In that case the expected syntax would have been:
/*eslint object-shorthand: "error"*/
/*eslint-env es6*/
var foo = {
w() {},
*x() {},
[y]() {},
z
};
This rule does not flag arrow functions inside of object literals. The following will not warn:
/*eslint object-shorthand: "error"*/
/*eslint-env es6*/
var foo = {
x: (y) => y
};
See Also:
-
no-useless-rename
which disallows renaming import, export, and destructured assignments to the same name.
Options
The rule takes an option which specifies when it should be applied. It can be set to one of the following values:
-
"always"
(default) expects that the shorthand will be used whenever possible. -
"methods"
ensures the method shorthand is used (also applies to generators). -
"properties"
ensures the property shorthand is used (where the key and variable name match). -
"never"
ensures that no property or method shorthand is used in any object literal. -
"consistent"
ensures that either all shorthand or all long-form will be used in an object literal. -
"consistent-as-needed"
ensures that either all shorthand or all long-form will be used in an object literal, but ensures all shorthand whenever possible.
You can set the option in configuration like this:
{
"object-shorthand": ["error", "always"]
}
Additionally, the rule takes an optional object configuration:
-
"avoidQuotes": true
indicates that long-form syntax is preferred whenever the object key is a string literal (default:false
). Note that this option can only be enabled when the string option is set to"always"
,"methods"
, or"properties"
. -
"ignoreConstructors": true
can be used to prevent the rule from reporting errors for constructor functions. (By default, the rule treats constructors the same way as other functions.) Note that this option can only be enabled when the string option is set to"always"
or"methods"
. -
"avoidExplicitReturnArrows": true
indicates that methods are preferred over explicit-return arrow functions for function properties. (By default, the rule allows either of these.) Note that this option can only be enabled when the string option is set to"always"
or"methods"
.
avoidQuotes
{
"object-shorthand": ["error", "always", { "avoidQuotes": true }]
}
Example of incorrect code for this rule with the "always", { "avoidQuotes": true }
option:
/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/
var foo = {
"bar-baz"() {}
};
Example of correct code for this rule with the "always", { "avoidQuotes": true }
option:
/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/
var foo = {
"bar-baz": function() {},
"qux": qux
};
ignoreConstructors
{
"object-shorthand": ["error", "always", { "ignoreConstructors": true }]
}
Example of correct code for this rule with the "always", { "ignoreConstructors": true }
option:
/*eslint object-shorthand: ["error", "always", { "ignoreConstructors": true }]*/
/*eslint-env es6*/
var foo = {
ConstructorFunction: function() {}
};
avoidExplicitReturnArrows
{
"object-shorthand": ["error", "always", { "avoidExplicitReturnArrows": true }]
}
Example of incorrect code for this rule with the "always", { "avoidExplicitReturnArrows": true }
option:
/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/
var foo = {
foo: (bar, baz) => {
return bar + baz;
},
qux: (foobar) => {
return foobar * 2;
}
};
Example of correct code for this rule with the "always", { "avoidExplicitReturnArrows": true }
option:
/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/
var foo = {
foo(bar, baz) {
return bar + baz;
},
qux: foobar => foobar * 2
};
Example of incorrect code for this rule with the "consistent"
option:
/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/
var foo = {
a,
b: "foo",
};
Examples of correct code for this rule with the "consistent"
option:
/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/
var foo = {
a: a,
b: "foo"
};
var bar = {
a,
b,
};
Example of incorrect code with the "consistent-as-needed"
option, which is very similar to "consistent"
:
/*eslint object-shorthand: [2, "consistent-as-needed"]*/
/*eslint-env es6*/
var foo = {
a: a,
b: b,
};
When Not To Use It
Anyone not yet in an ES6 environment would not want to apply this rule. Others may find the terseness of the shorthand syntax harder to read and may not want to encourage it with this rule.
Further Reading
Object initializer - MDN Source: http://eslint.org/docs/rules/
Unnecessarily quoted property 'Hourly' found. Open
'Hourly': hourlyTimeOptions,
- Read upRead up
- Create a ticketCreate a ticket
- Exclude checks
require quotes around object literal property names (quote-props)
Object literal property names can be defined in two ways: using literals or using strings. For example, these two objects are equivalent:
var object1 = {
property: true
};
var object2 = {
"property": true
};
In many cases, it doesn't matter if you choose to use an identifier instead of a string or vice-versa. Even so, you might decide to enforce a consistent style in your code.
There are, however, some occasions when you must use quotes:
- If you are using an ECMAScript 3 JavaScript engine (such as IE8) and you want to use a keyword (such as
if
) as a property name. This restriction was removed in ECMAScript 5. - You want to use a non-identifier character in your property name, such as having a property with a space like
"one two"
.
Another example where quotes do matter is when using numeric literals as property keys:
var object = {
1e2: 1,
100: 2
};
This may look alright at first sight, but this code in fact throws a syntax error in ECMAScript 5 strict mode. This happens because 1e2
and 100
are coerced into strings before getting used as the property name. Both String(1e2)
and String(100)
happen to be equal to "100"
, which causes the "Duplicate data property in object literal not allowed in strict mode" error. Issues like that can be tricky to debug, so some prefer to require quotes around all property names.
Rule Details
This rule requires quotes around object literal property names.
Options
This rule has two options, a string option and an object option.
String option:
-
"always"
(default) requires quotes around all object literal property names -
"as-needed"
disallows quotes around object literal property names that are not strictly required -
"consistent"
enforces a consistent quote style; in a given object, either all of the properties should be quoted, or none of the properties should be quoted -
"consistent-as-needed"
requires quotes around all object literal property names if any name strictly requires quotes, otherwise disallows quotes around object property names
Object option:
-
"keywords": true
requires quotes around language keywords used as object property names (only applies when usingas-needed
orconsistent-as-needed
) -
"unnecessary": true
(default) disallows quotes around object literal property names that are not strictly required (only applies when usingas-needed
) -
"unnecessary": false
allows quotes around object literal property names that are not strictly required (only applies when usingas-needed
) -
"numbers": true
requires quotes around numbers used as object property names (only applies when usingas-needed
)
always
Examples of incorrect code for this rule with the default "always"
option:
/*eslint quote-props: ["error", "always"]*/
var object = {
foo: "bar",
baz: 42,
"qux-lorem": true
};
Examples of correct code for this rule with the default "always"
option:
/*eslint quote-props: ["error", "always"]*/
/*eslint-env es6*/
var object1 = {
"foo": "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
'foo': 'bar',
'baz': 42,
'qux-lorem': true
};
var object3 = {
foo() {
return;
}
};
as-needed
Examples of incorrect code for this rule with the "as-needed"
option:
/*eslint quote-props: ["error", "as-needed"]*/
var object = {
"a": 0,
"0": 0,
"true": 0,
"null": 0
};
Examples of correct code for this rule with the "as-needed"
option:
/*eslint quote-props: ["error", "as-needed"]*/
/*eslint-env es6*/
var object1 = {
"a-b": 0,
"0x0": 0,
"1e2": 0
};
var object2 = {
foo: 'bar',
baz: 42,
true: 0,
0: 0,
'qux-lorem': true
};
var object3 = {
foo() {
return;
}
};
consistent
Examples of incorrect code for this rule with the "consistent"
option:
/*eslint quote-props: ["error", "consistent"]*/
var object1 = {
foo: "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
'foo': 'bar',
baz: 42
};
Examples of correct code for this rule with the "consistent"
option:
/*eslint quote-props: ["error", "consistent"]*/
var object1 = {
"foo": "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
'foo': 'bar',
'baz': 42
};
var object3 = {
foo: 'bar',
baz: 42
};
consistent-as-needed
Examples of incorrect code for this rule with the "consistent-as-needed"
option:
/*eslint quote-props: ["error", "consistent-as-needed"]*/
var object1 = {
foo: "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
'foo': 'bar',
'baz': 42
};
Examples of correct code for this rule with the "consistent-as-needed"
option:
/*eslint quote-props: ["error", "consistent-as-needed"]*/
var object1 = {
"foo": "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
foo: 'bar',
baz: 42
};
keywords
Examples of additional incorrect code for this rule with the "as-needed", { "keywords": true }
options:
/*eslint quote-props: ["error", "as-needed", { "keywords": true }]*/
var x = {
while: 1,
volatile: "foo"
};
Examples of additional incorrect code for this rule with the "consistent-as-needed", { "keywords": true }
options:
/*eslint quote-props: ["error", "consistent-as-needed", { "keywords": true }]*/
var x = {
"prop": 1,
"bar": "foo"
};
unnecessary
Examples of additional correct code for this rule with the "as-needed", { "unnecessary": false }
options:
/*eslint quote-props: ["error", "as-needed", { "keywords": true, "unnecessary": false }]*/
var x = {
"while": 1,
"foo": "bar" // Would normally have caused a warning
};
numbers
Examples of additional incorrect code for this rule with the "as-needed", { "numbers": true }
options:
/*eslint quote-props: ["error", "as-needed", { "numbers": true }]*/
var x = {
100: 1
}
When Not To Use It
If you don't care if property names are consistently wrapped in quotes or not, and you don't target legacy ES3 environments, turn this rule off.
Further Reading
Unexpected var, use let or const instead. Open
var timeData = [];
- Read upRead up
- Create a ticketCreate a ticket
- Exclude checks
require let
or const
instead of var
(no-var)
ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let
and const
keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes
such as:
var count = people.length;
var enoughFood = count > sandwiches.length;
if (enoughFood) {
var count = sandwiches.length; // accidentally overriding the count variable
console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}
// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");
Rule Details
This rule is aimed at discouraging the use of var
and encouraging the use of const
or let
instead.
Examples
Examples of incorrect code for this rule:
/*eslint no-var: "error"*/
var x = "y";
var CONFIG = {};
Examples of correct code for this rule:
/*eslint no-var: "error"*/
/*eslint-env es6*/
let x = "y";
const CONFIG = {};
When Not To Use It
In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their
codebase may not want to apply this rule if the cost of migrating from var
to let
is too costly.
Source: http://eslint.org/docs/rules/
Unnecessarily quoted property 'Once' found. Open
'Once': [],
- Read upRead up
- Create a ticketCreate a ticket
- Exclude checks
require quotes around object literal property names (quote-props)
Object literal property names can be defined in two ways: using literals or using strings. For example, these two objects are equivalent:
var object1 = {
property: true
};
var object2 = {
"property": true
};
In many cases, it doesn't matter if you choose to use an identifier instead of a string or vice-versa. Even so, you might decide to enforce a consistent style in your code.
There are, however, some occasions when you must use quotes:
- If you are using an ECMAScript 3 JavaScript engine (such as IE8) and you want to use a keyword (such as
if
) as a property name. This restriction was removed in ECMAScript 5. - You want to use a non-identifier character in your property name, such as having a property with a space like
"one two"
.
Another example where quotes do matter is when using numeric literals as property keys:
var object = {
1e2: 1,
100: 2
};
This may look alright at first sight, but this code in fact throws a syntax error in ECMAScript 5 strict mode. This happens because 1e2
and 100
are coerced into strings before getting used as the property name. Both String(1e2)
and String(100)
happen to be equal to "100"
, which causes the "Duplicate data property in object literal not allowed in strict mode" error. Issues like that can be tricky to debug, so some prefer to require quotes around all property names.
Rule Details
This rule requires quotes around object literal property names.
Options
This rule has two options, a string option and an object option.
String option:
-
"always"
(default) requires quotes around all object literal property names -
"as-needed"
disallows quotes around object literal property names that are not strictly required -
"consistent"
enforces a consistent quote style; in a given object, either all of the properties should be quoted, or none of the properties should be quoted -
"consistent-as-needed"
requires quotes around all object literal property names if any name strictly requires quotes, otherwise disallows quotes around object property names
Object option:
-
"keywords": true
requires quotes around language keywords used as object property names (only applies when usingas-needed
orconsistent-as-needed
) -
"unnecessary": true
(default) disallows quotes around object literal property names that are not strictly required (only applies when usingas-needed
) -
"unnecessary": false
allows quotes around object literal property names that are not strictly required (only applies when usingas-needed
) -
"numbers": true
requires quotes around numbers used as object property names (only applies when usingas-needed
)
always
Examples of incorrect code for this rule with the default "always"
option:
/*eslint quote-props: ["error", "always"]*/
var object = {
foo: "bar",
baz: 42,
"qux-lorem": true
};
Examples of correct code for this rule with the default "always"
option:
/*eslint quote-props: ["error", "always"]*/
/*eslint-env es6*/
var object1 = {
"foo": "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
'foo': 'bar',
'baz': 42,
'qux-lorem': true
};
var object3 = {
foo() {
return;
}
};
as-needed
Examples of incorrect code for this rule with the "as-needed"
option:
/*eslint quote-props: ["error", "as-needed"]*/
var object = {
"a": 0,
"0": 0,
"true": 0,
"null": 0
};
Examples of correct code for this rule with the "as-needed"
option:
/*eslint quote-props: ["error", "as-needed"]*/
/*eslint-env es6*/
var object1 = {
"a-b": 0,
"0x0": 0,
"1e2": 0
};
var object2 = {
foo: 'bar',
baz: 42,
true: 0,
0: 0,
'qux-lorem': true
};
var object3 = {
foo() {
return;
}
};
consistent
Examples of incorrect code for this rule with the "consistent"
option:
/*eslint quote-props: ["error", "consistent"]*/
var object1 = {
foo: "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
'foo': 'bar',
baz: 42
};
Examples of correct code for this rule with the "consistent"
option:
/*eslint quote-props: ["error", "consistent"]*/
var object1 = {
"foo": "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
'foo': 'bar',
'baz': 42
};
var object3 = {
foo: 'bar',
baz: 42
};
consistent-as-needed
Examples of incorrect code for this rule with the "consistent-as-needed"
option:
/*eslint quote-props: ["error", "consistent-as-needed"]*/
var object1 = {
foo: "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
'foo': 'bar',
'baz': 42
};
Examples of correct code for this rule with the "consistent-as-needed"
option:
/*eslint quote-props: ["error", "consistent-as-needed"]*/
var object1 = {
"foo": "bar",
"baz": 42,
"qux-lorem": true
};
var object2 = {
foo: 'bar',
baz: 42
};
keywords
Examples of additional incorrect code for this rule with the "as-needed", { "keywords": true }
options:
/*eslint quote-props: ["error", "as-needed", { "keywords": true }]*/
var x = {
while: 1,
volatile: "foo"
};
Examples of additional incorrect code for this rule with the "consistent-as-needed", { "keywords": true }
options:
/*eslint quote-props: ["error", "consistent-as-needed", { "keywords": true }]*/
var x = {
"prop": 1,
"bar": "foo"
};
unnecessary
Examples of additional correct code for this rule with the "as-needed", { "unnecessary": false }
options:
/*eslint quote-props: ["error", "as-needed", { "keywords": true, "unnecessary": false }]*/
var x = {
"while": 1,
"foo": "bar" // Would normally have caused a warning
};
numbers
Examples of additional incorrect code for this rule with the "as-needed", { "numbers": true }
options:
/*eslint quote-props: ["error", "as-needed", { "numbers": true }]*/
var x = {
100: 1
}
When Not To Use It
If you don't care if property names are consistently wrapped in quotes or not, and you don't target legacy ES3 environments, turn this rule off.
Further Reading
Unexpected var, use let or const instead. Open
var timeDataBuilder = function(timeType, iterations) {
- Read upRead up
- Create a ticketCreate a ticket
- Exclude checks
require let
or const
instead of var
(no-var)
ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let
and const
keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes
such as:
var count = people.length;
var enoughFood = count > sandwiches.length;
if (enoughFood) {
var count = sandwiches.length; // accidentally overriding the count variable
console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}
// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");
Rule Details
This rule is aimed at discouraging the use of var
and encouraging the use of const
or let
instead.
Examples
Examples of incorrect code for this rule:
/*eslint no-var: "error"*/
var x = "y";
var CONFIG = {};
Examples of correct code for this rule:
/*eslint no-var: "error"*/
/*eslint-env es6*/
let x = "y";
const CONFIG = {};
When Not To Use It
In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their
codebase may not want to apply this rule if the cost of migrating from var
to let
is too costly.
Source: http://eslint.org/docs/rules/
Unexpected var, use let or const instead. Open
for (var value = 1; value <= iterations; value++) {
- Read upRead up
- Create a ticketCreate a ticket
- Exclude checks
require let
or const
instead of var
(no-var)
ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let
and const
keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes
such as:
var count = people.length;
var enoughFood = count > sandwiches.length;
if (enoughFood) {
var count = sandwiches.length; // accidentally overriding the count variable
console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}
// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");
Rule Details
This rule is aimed at discouraging the use of var
and encouraging the use of const
or let
instead.
Examples
Examples of incorrect code for this rule:
/*eslint no-var: "error"*/
var x = "y";
var CONFIG = {};
Examples of correct code for this rule:
/*eslint no-var: "error"*/
/*eslint-env es6*/
let x = "y";
const CONFIG = {};
When Not To Use It
In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their
codebase may not want to apply this rule if the cost of migrating from var
to let
is too costly.
Source: http://eslint.org/docs/rules/
Unary operator '++' used. Open
for (var value = 1; value <= iterations; value++) {
- Read upRead up
- Create a ticketCreate a ticket
- Exclude checks
disallow the unary operators ++
and --
(no-plusplus)
Because the unary ++
and --
operators are subject to automatic semicolon insertion, differences in whitespace can change semantics of source code.
var i = 10;
var j = 20;
i ++
j
// i = 11, j = 20
var i = 10;
var j = 20;
i
++
j
// i = 10, j = 21
Rule Details
This rule disallows the unary operators ++
and --
.
Examples of incorrect code for this rule:
/*eslint no-plusplus: "error"*/
var foo = 0;
foo++;
var bar = 42;
bar--;
for (i = 0; i < l; i++) {
return;
}
Examples of correct code for this rule:
/*eslint no-plusplus: "error"*/
var foo = 0;
foo += 1;
var bar = 42;
bar -= 1;
for (i = 0; i < l; i += 1) {
return;
}
Options
This rule has an object option.
-
"allowForLoopAfterthoughts": true
allows unary operators++
and--
in the afterthought (final expression) of afor
loop.
allowForLoopAfterthoughts
Examples of correct code for this rule with the { "allowForLoopAfterthoughts": true }
option:
/*eslint no-plusplus: ["error", { "allowForLoopAfterthoughts": true }]*/
for (i = 0; i < l; i++) {
return;
}
for (i = 0; i < l; i--) {
return;
}
Source: http://eslint.org/docs/rules/