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'tags' is never reassigned. Use 'const' instead. Open
let tags = node.frontmatter.tags
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Suggest using const
(prefer-const)
If a variable is never reassigned, using the const
declaration is better.
const
declaration tells readers, "this variable is never reassigned," reducing cognitive load and improving maintainability.
Rule Details
This rule is aimed at flagging variables that are declared using let
keyword, but never reassigned after the initial assignment.
Examples of incorrect code for this rule:
/*eslint prefer-const: "error"*/
/*eslint-env es6*/
// it's initialized and never reassigned.
let a = 3;
console.log(a);
let a;
a = 0;
console.log(a);
// `i` is redefined (not reassigned) on each loop step.
for (let i in [1, 2, 3]) {
console.log(i);
}
// `a` is redefined (not reassigned) on each loop step.
for (let a of [1, 2, 3]) {
console.log(a);
}
Examples of correct code for this rule:
/*eslint prefer-const: "error"*/
/*eslint-env es6*/
// using const.
const a = 0;
// it's never initialized.
let a;
console.log(a);
// it's reassigned after initialized.
let a;
a = 0;
a = 1;
console.log(a);
// it's initialized in a different block from the declaration.
let a;
if (true) {
a = 0;
}
console.log(a);
// it's initialized at a place that we cannot write a variable declaration.
let a;
if (true) a = 0;
console.log(a);
// `i` gets a new binding each iteration
for (const i in [1, 2, 3]) {
console.log(i);
}
// `a` gets a new binding each iteration
for (const a of [1, 2, 3]) {
console.log(a);
}
// `end` is never reassigned, but we cannot separate the declarations without modifying the scope.
for (let i = 0, end = 10; i < end; ++i) {
console.log(a);
}
// suggest to use `no-var` rule.
var b = 3;
console.log(b);
Options
{
"prefer-const": ["error", {
"destructuring": "any",
"ignoreReadBeforeAssign": false
}]
}
destructuring
The kind of the way to address variables in destructuring. There are 2 values:
-
"any"
(default) - If any variables in destructuring should beconst
, this rule warns for those variables. -
"all"
- If all variables in destructuring should beconst
, this rule warns the variables. Otherwise, ignores them.
Examples of incorrect code for the default {"destructuring": "any"}
option:
/*eslint prefer-const: "error"*/
/*eslint-env es6*/
let {a, b} = obj; /*error 'b' is never reassigned, use 'const' instead.*/
a = a + 1;
Examples of correct code for the default {"destructuring": "any"}
option:
/*eslint prefer-const: "error"*/
/*eslint-env es6*/
// using const.
const {a: a0, b} = obj;
const a = a0 + 1;
// all variables are reassigned.
let {a, b} = obj;
a = a + 1;
b = b + 1;
Examples of incorrect code for the {"destructuring": "all"}
option:
/*eslint prefer-const: ["error", {"destructuring": "all"}]*/
/*eslint-env es6*/
// all of `a` and `b` should be const, so those are warned.
let {a, b} = obj; /*error 'a' is never reassigned, use 'const' instead.
'b' is never reassigned, use 'const' instead.*/
Examples of correct code for the {"destructuring": "all"}
option:
/*eslint prefer-const: ["error", {"destructuring": "all"}]*/
/*eslint-env es6*/
// 'b' is never reassigned, but all of `a` and `b` should not be const, so those are ignored.
let {a, b} = obj;
a = a + 1;
ignoreReadBeforeAssign
This is an option to avoid conflicting with no-use-before-define
rule (without "nofunc"
option).
If true
is specified, this rule will ignore variables that are read between the declaration and the first assignment.
Default is false
.
Examples of correct code for the {"ignoreReadBeforeAssign": true}
option:
/*eslint prefer-const: ["error", {"ignoreReadBeforeAssign": true}]*/
/*eslint-env es6*/
let timer;
function initialize() {
if (foo()) {
clearInterval(timer);
}
}
timer = setInterval(initialize, 100);
Examples of correct code for the default {"ignoreReadBeforeAssign": false}
option:
/*eslint prefer-const: ["error", {"ignoreReadBeforeAssign": false}]*/
/*eslint-env es6*/
const timer = setInterval(initialize, 100);
function initialize() {
if (foo()) {
clearInterval(timer);
}
}
When Not To Use It
If you don't want to be notified about variables that are never reassigned after initial assignment, you can safely disable this rule.
Related Rules
- [no-var](no-var.md)
- [no-use-before-define](no-use-before-define.md) Source: http://eslint.org/docs/rules/
'previous' is never reassigned. Use 'const' instead. Open
let previous =
- Read upRead up
- Exclude checks
Suggest using const
(prefer-const)
If a variable is never reassigned, using the const
declaration is better.
const
declaration tells readers, "this variable is never reassigned," reducing cognitive load and improving maintainability.
Rule Details
This rule is aimed at flagging variables that are declared using let
keyword, but never reassigned after the initial assignment.
Examples of incorrect code for this rule:
/*eslint prefer-const: "error"*/
/*eslint-env es6*/
// it's initialized and never reassigned.
let a = 3;
console.log(a);
let a;
a = 0;
console.log(a);
// `i` is redefined (not reassigned) on each loop step.
for (let i in [1, 2, 3]) {
console.log(i);
}
// `a` is redefined (not reassigned) on each loop step.
for (let a of [1, 2, 3]) {
console.log(a);
}
Examples of correct code for this rule:
/*eslint prefer-const: "error"*/
/*eslint-env es6*/
// using const.
const a = 0;
// it's never initialized.
let a;
console.log(a);
// it's reassigned after initialized.
let a;
a = 0;
a = 1;
console.log(a);
// it's initialized in a different block from the declaration.
let a;
if (true) {
a = 0;
}
console.log(a);
// it's initialized at a place that we cannot write a variable declaration.
let a;
if (true) a = 0;
console.log(a);
// `i` gets a new binding each iteration
for (const i in [1, 2, 3]) {
console.log(i);
}
// `a` gets a new binding each iteration
for (const a of [1, 2, 3]) {
console.log(a);
}
// `end` is never reassigned, but we cannot separate the declarations without modifying the scope.
for (let i = 0, end = 10; i < end; ++i) {
console.log(a);
}
// suggest to use `no-var` rule.
var b = 3;
console.log(b);
Options
{
"prefer-const": ["error", {
"destructuring": "any",
"ignoreReadBeforeAssign": false
}]
}
destructuring
The kind of the way to address variables in destructuring. There are 2 values:
-
"any"
(default) - If any variables in destructuring should beconst
, this rule warns for those variables. -
"all"
- If all variables in destructuring should beconst
, this rule warns the variables. Otherwise, ignores them.
Examples of incorrect code for the default {"destructuring": "any"}
option:
/*eslint prefer-const: "error"*/
/*eslint-env es6*/
let {a, b} = obj; /*error 'b' is never reassigned, use 'const' instead.*/
a = a + 1;
Examples of correct code for the default {"destructuring": "any"}
option:
/*eslint prefer-const: "error"*/
/*eslint-env es6*/
// using const.
const {a: a0, b} = obj;
const a = a0 + 1;
// all variables are reassigned.
let {a, b} = obj;
a = a + 1;
b = b + 1;
Examples of incorrect code for the {"destructuring": "all"}
option:
/*eslint prefer-const: ["error", {"destructuring": "all"}]*/
/*eslint-env es6*/
// all of `a` and `b` should be const, so those are warned.
let {a, b} = obj; /*error 'a' is never reassigned, use 'const' instead.
'b' is never reassigned, use 'const' instead.*/
Examples of correct code for the {"destructuring": "all"}
option:
/*eslint prefer-const: ["error", {"destructuring": "all"}]*/
/*eslint-env es6*/
// 'b' is never reassigned, but all of `a` and `b` should not be const, so those are ignored.
let {a, b} = obj;
a = a + 1;
ignoreReadBeforeAssign
This is an option to avoid conflicting with no-use-before-define
rule (without "nofunc"
option).
If true
is specified, this rule will ignore variables that are read between the declaration and the first assignment.
Default is false
.
Examples of correct code for the {"ignoreReadBeforeAssign": true}
option:
/*eslint prefer-const: ["error", {"ignoreReadBeforeAssign": true}]*/
/*eslint-env es6*/
let timer;
function initialize() {
if (foo()) {
clearInterval(timer);
}
}
timer = setInterval(initialize, 100);
Examples of correct code for the default {"ignoreReadBeforeAssign": false}
option:
/*eslint prefer-const: ["error", {"ignoreReadBeforeAssign": false}]*/
/*eslint-env es6*/
const timer = setInterval(initialize, 100);
function initialize() {
if (foo()) {
clearInterval(timer);
}
}
When Not To Use It
If you don't want to be notified about variables that are never reassigned after initial assignment, you can safely disable this rule.
Related Rules
- [no-var](no-var.md)
- [no-use-before-define](no-use-before-define.md) Source: http://eslint.org/docs/rules/
Expected to return a value in arrow function. Open
postNodes.map(node => {
- Read upRead up
- Exclude checks
Enforces return statements in callbacks of array's methods (array-callback-return)
Array
has several methods for filtering, mapping, and folding.
If we forget to write return
statement in a callback of those, it's probably a mistake.
// example: convert ['a', 'b', 'c'] --> {a: 0, b: 1, c: 2}
var indexMap = myArray.reduce(function(memo, item, index) {
memo[item] = index;
}, {}); // Error: cannot set property 'b' of undefined
This rule enforces usage of return
statement in callbacks of array's methods.
Rule Details
This rule finds callback functions of the following methods, then checks usage of return
statement.
Array.from
Array.prototype.every
Array.prototype.filter
Array.prototype.find
Array.prototype.findIndex
Array.prototype.map
Array.prototype.reduce
Array.prototype.reduceRight
Array.prototype.some
Array.prototype.sort
- And above of typed arrays.
Examples of incorrect code for this rule:
/*eslint array-callback-return: "error"*/
var indexMap = myArray.reduce(function(memo, item, index) {
memo[item] = index;
}, {});
var foo = Array.from(nodes, function(node) {
if (node.tagName === "DIV") {
return true;
}
});
var bar = foo.filter(function(x) {
if (x) {
return true;
} else {
return;
}
});
Examples of correct code for this rule:
/*eslint array-callback-return: "error"*/
var indexMap = myArray.reduce(function(memo, item, index) {
memo[item] = index;
return memo;
}, {});
var foo = Array.from(nodes, function(node) {
if (node.tagName === "DIV") {
return true;
}
return false;
});
var bar = foo.map(node => node.getAttribute("id"));
Options
This rule has an object option:
-
"allowImplicit": false
(default) When set to true, allows implicitly returningundefined
with areturn
statement containing no expression.
Examples of correct code for the { "allowImplicit": true }
option:
/*eslint array-callback-return: ["error", { allowImplicit: true }]*/
var undefAllTheThings = myArray.map(function(item) {
return;
});
Known Limitations
This rule checks callback functions of methods with the given names, even if the object which has the method is not an array.
When Not To Use It
If you don't want to warn about usage of return
statement in callbacks of array's methods, then it's safe to disable this rule.
Source: http://eslint.org/docs/rules/
'path' is defined but never used. Open
transformHeaders: (headers, path) => headers, // optional transform for manipulating headers under each path (e.g.sorting), etc.
- Read upRead up
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Disallow Unused Variables (no-unused-vars)
Variables that are declared and not used anywhere in the code are most likely an error due to incomplete refactoring. Such variables take up space in the code and can lead to confusion by readers.
Rule Details
This rule is aimed at eliminating unused variables, functions, and parameters of functions.
A variable is considered to be used if any of the following are true:
- It represents a function that is called (
doSomething()
) - It is read (
var y = x
) - It is passed into a function as an argument (
doSomething(x)
) - It is read inside of a function that is passed to another function (
doSomething(function() { foo(); })
)
A variable is not considered to be used if it is only ever assigned to (var x = 5
) or declared.
Examples of incorrect code for this rule:
/*eslint no-unused-vars: "error"*/
/*global some_unused_var*/
// It checks variables you have defined as global
some_unused_var = 42;
var x;
// Write-only variables are not considered as used.
var y = 10;
y = 5;
// A read for a modification of itself is not considered as used.
var z = 0;
z = z + 1;
// By default, unused arguments cause warnings.
(function(foo) {
return 5;
})();
// Unused recursive functions also cause warnings.
function fact(n) {
if (n < 2) return 1;
return n * fact(n - 1);
}
// When a function definition destructures an array, unused entries from the array also cause warnings.
function getY([x, y]) {
return y;
}
Examples of correct code for this rule:
/*eslint no-unused-vars: "error"*/
var x = 10;
alert(x);
// foo is considered used here
myFunc(function foo() {
// ...
}.bind(this));
(function(foo) {
return foo;
})();
var myFunc;
myFunc = setTimeout(function() {
// myFunc is considered used
myFunc();
}, 50);
// Only the second argument from the descructured array is used.
function getY([, y]) {
return y;
}
exported
In environments outside of CommonJS or ECMAScript modules, you may use var
to create a global variable that may be used by other scripts. You can use the /* exported variableName */
comment block to indicate that this variable is being exported and therefore should not be considered unused.
Note that /* exported */
has no effect for any of the following:
- when the environment is
node
orcommonjs
- when
parserOptions.sourceType
ismodule
- when
ecmaFeatures.globalReturn
istrue
The line comment // exported variableName
will not work as exported
is not line-specific.
Examples of correct code for /* exported variableName */
operation:
/* exported global_var */
var global_var = 42;
Options
This rule takes one argument which can be a string or an object. The string settings are the same as those of the vars
property (explained below).
By default this rule is enabled with all
option for variables and after-used
for arguments.
{
"rules": {
"no-unused-vars": ["error", { "vars": "all", "args": "after-used", "ignoreRestSiblings": false }]
}
}
vars
The vars
option has two settings:
-
all
checks all variables for usage, including those in the global scope. This is the default setting. -
local
checks only that locally-declared variables are used but will allow global variables to be unused.
vars: local
Examples of correct code for the { "vars": "local" }
option:
/*eslint no-unused-vars: ["error", { "vars": "local" }]*/
/*global some_unused_var */
some_unused_var = 42;
varsIgnorePattern
The varsIgnorePattern
option specifies exceptions not to check for usage: variables whose names match a regexp pattern. For example, variables whose names contain ignored
or Ignored
.
Examples of correct code for the { "varsIgnorePattern": "[iI]gnored" }
option:
/*eslint no-unused-vars: ["error", { "varsIgnorePattern": "[iI]gnored" }]*/
var firstVarIgnored = 1;
var secondVar = 2;
console.log(secondVar);
args
The args
option has three settings:
-
after-used
- only the last argument must be used. This allows you, for instance, to have two named parameters to a function and as long as you use the second argument, ESLint will not warn you about the first. This is the default setting. -
all
- all named arguments must be used. -
none
- do not check arguments.
args: after-used
Examples of incorrect code for the default { "args": "after-used" }
option:
/*eslint no-unused-vars: ["error", { "args": "after-used" }]*/
// 1 error
// "baz" is defined but never used
(function(foo, bar, baz) {
return bar;
})();
Examples of correct code for the default { "args": "after-used" }
option:
/*eslint no-unused-vars: ["error", {"args": "after-used"}]*/
(function(foo, bar, baz) {
return baz;
})();
args: all
Examples of incorrect code for the { "args": "all" }
option:
/*eslint no-unused-vars: ["error", { "args": "all" }]*/
// 2 errors
// "foo" is defined but never used
// "baz" is defined but never used
(function(foo, bar, baz) {
return bar;
})();
args: none
Examples of correct code for the { "args": "none" }
option:
/*eslint no-unused-vars: ["error", { "args": "none" }]*/
(function(foo, bar, baz) {
return bar;
})();
ignoreRestSiblings
The ignoreRestSiblings
option is a boolean (default: false
). Using a Rest Property it is possible to "omit" properties from an object, but by default the sibling properties are marked as "unused". With this option enabled the rest property's siblings are ignored.
Examples of correct code for the { "ignoreRestSiblings": true }
option:
/*eslint no-unused-vars: ["error", { "ignoreRestSiblings": true }]*/
// 'type' is ignored because it has a rest property sibling.
var { type, ...coords } = data;
argsIgnorePattern
The argsIgnorePattern
option specifies exceptions not to check for usage: arguments whose names match a regexp pattern. For example, variables whose names begin with an underscore.
Examples of correct code for the { "argsIgnorePattern": "^_" }
option:
/*eslint no-unused-vars: ["error", { "argsIgnorePattern": "^_" }]*/
function foo(x, _y) {
return x + 1;
}
foo();
caughtErrors
The caughtErrors
option is used for catch
block arguments validation.
It has two settings:
-
none
- do not check error objects. This is the default setting. -
all
- all named arguments must be used.
caughtErrors: none
Not specifying this rule is equivalent of assigning it to none
.
Examples of correct code for the { "caughtErrors": "none" }
option:
/*eslint no-unused-vars: ["error", { "caughtErrors": "none" }]*/
try {
//...
} catch (err) {
console.error("errors");
}
caughtErrors: all
Examples of incorrect code for the { "caughtErrors": "all" }
option:
/*eslint no-unused-vars: ["error", { "caughtErrors": "all" }]*/
// 1 error
// "err" is defined but never used
try {
//...
} catch (err) {
console.error("errors");
}
caughtErrorsIgnorePattern
The caughtErrorsIgnorePattern
option specifies exceptions not to check for usage: catch arguments whose names match a regexp pattern. For example, variables whose names begin with a string 'ignore'.
Examples of correct code for the { "caughtErrorsIgnorePattern": "^ignore" }
option:
/*eslint no-unused-vars: ["error", { "caughtErrorsIgnorePattern": "^ignore" }]*/
try {
//...
} catch (ignoreErr) {
console.error("errors");
}
When Not To Use It
If you don't want to be notified about unused variables or function arguments, you can safely turn this rule off. Source: http://eslint.org/docs/rules/
Use object destructuring. Open
let edges = result.data.allMarkdownRemark.edges
- Read upRead up
- Exclude checks
Prefer destructuring from arrays and objects (prefer-destructuring)
With JavaScript ES6, a new syntax was added for creating variables from an array index or object property, called destructuring. This rule enforces usage of destructuring instead of accessing a property through a member expression.
Rule Details
Options
This rule takes two sets of configuration objects. The first object parameter determines what types of destructuring the rule applies to.
The two properties, array
and object
, can be used to turn on or off the destructuring requirement for each of those types independently. By default, both are true.
Alternatively, you can use separate configurations for different assignment types. It accepts 2 other keys instead of array
and object
.
One key is VariableDeclarator
and the other is AssignmentExpression
, which can be used to control the destructuring requirement for each of those types independently. Each property accepts an object that accepts two properties, array
and object
, which can be used to control the destructuring requirement for each of array
and object
independently for variable declarations and assignment expressions. By default, array
and object
are set to true for both VariableDeclarator
and AssignmentExpression
.
The rule has a second object with a single key, enforceForRenamedProperties
, which determines whether the object
destructuring applies to renamed variables.
Examples of incorrect code for this rule:
// With `array` enabled
var foo = array[0];
// With `object` enabled
var foo = object.foo;
var foo = object['foo'];
Examples of correct code for this rule:
// With `array` enabled
var [ foo ] = array;
var foo = array[someIndex];
// With `object` enabled
var { foo } = object;
var foo = object.bar;
let foo;
({ foo } = object);
Examples of incorrect code when enforceForRenamedProperties
is enabled:
var foo = object.bar;
Examples of correct code when enforceForRenamedProperties
is enabled:
var { bar: foo } = object;
An example configuration, with the defaults array
and object
filled in, looks like this:
{
"rules": {
"prefer-destructuring": ["error", {
"array": true,
"object": true
}, {
"enforceForRenamedProperties": false
}]
}
}
The two properties, array
and object
, which can be used to turn on or off the destructuring requirement for each of those types independently. By default, both are true.
For example, the following configuration enforces only object destructuring, but not array destructuring:
{
"rules": {
"prefer-destructuring": ["error", {"object": true, "array": false}]
}
}
An example configuration, with the defaults VariableDeclarator
and AssignmentExpression
filled in, looks like this:
{
"rules": {
"prefer-destructuring": ["error", {
"VariableDeclarator": {
"array": false,
"object": true
},
"AssignmentExpression": {
"array": true,
"object": true
}
}, {
"enforceForRenamedProperties": false
}]
}
}
The two properties, VariableDeclarator
and AssignmentExpression
, which can be used to turn on or off the destructuring requirement for array
and object
. By default, all values are true.
For example, the following configuration enforces object destructuring in variable declarations and enforces array destructuring in assignment expressions.
{
"rules": {
"prefer-destructuring": ["error", {
"VariableDeclarator": {
"array": false,
"object": true
},
"AssignmentExpression": {
"array": true,
"object": false
}
}, {
"enforceForRenamedProperties": false
}]
}
}
Examples of correct code when object destructuring in VariableDeclarator
is enforced:
/* eslint prefer-destructuring: ["error", {VariableDeclarator: {object: true}}] */
var {bar: foo} = object;
Examples of correct code when array destructuring in AssignmentExpression
is enforced:
/* eslint prefer-destructuring: ["error", {AssignmentExpression: {array: true}}] */
[bar] = array;
When Not To Use It
If you want to be able to access array indices or object properties directly, you can either configure the rule to your tastes or disable the rule entirely.
Additionally, if you intend to access large array indices directly, like:
var foo = array[100];
Then the array
part of this rule is not recommended, as destructuring does not match this use case very well.
Or for non-iterable 'array-like' objects:
var $ = require('jquery');
var foo = $('body')[0];
var [bar] = $('body'); // fails with a TypeError
Further Reading
If you want to learn more about destructuring, check out the links below: