Showing 1,940 of 1,940 total issues
Unreachable code. Open
break;
- Read upRead up
- Exclude checks
disallow unreachable code after return
, throw
, continue
, and break
statements (no-unreachable)
Because the return
, throw
, break
, and continue
statements unconditionally exit a block of code, any statements after them cannot be executed. Unreachable statements are usually a mistake.
function fn() {
x = 1;
return x;
x = 3; // this will never execute
}
Rule Details
This rule disallows unreachable code after return
, throw
, continue
, and break
statements.
Examples of incorrect code for this rule:
/*eslint no-unreachable: "error"*/
function foo() {
return true;
console.log("done");
}
function bar() {
throw new Error("Oops!");
console.log("done");
}
while(value) {
break;
console.log("done");
}
throw new Error("Oops!");
console.log("done");
function baz() {
if (Math.random() < 0.5) {
return;
} else {
throw new Error();
}
console.log("done");
}
for (;;) {}
console.log("done");
Examples of correct code for this rule, because of JavaScript function and variable hoisting:
/*eslint no-unreachable: "error"*/
function foo() {
return bar();
function bar() {
return 1;
}
}
function bar() {
return x;
var x;
}
switch (foo) {
case 1:
break;
var x;
}
Source: http://eslint.org/docs/rules/
Expected '===' and instead saw '=='. Open
if (typeof(json) == "string") {
- Read upRead up
- Exclude checks
Require === and !== (eqeqeq)
It is considered good practice to use the type-safe equality operators ===
and !==
instead of their regular counterparts ==
and !=
.
The reason for this is that ==
and !=
do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm.
For instance, the following statements are all considered true
:
[] == false
[] == ![]
3 == "03"
If one of those occurs in an innocent-looking statement such as a == b
the actual problem is very difficult to spot.
Rule Details
This rule is aimed at eliminating the type-unsafe equality operators.
Examples of incorrect code for this rule:
/*eslint eqeqeq: "error"*/
if (x == 42) { }
if ("" == text) { }
if (obj.getStuff() != undefined) { }
The --fix
option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof
expression, or if both operands are literals with the same type.
Options
always
The "always"
option (default) enforces the use of ===
and !==
in every situation (except when you opt-in to more specific handling of null
[see below]).
Examples of incorrect code for the "always"
option:
/*eslint eqeqeq: ["error", "always"]*/
a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null
Examples of correct code for the "always"
option:
/*eslint eqeqeq: ["error", "always"]*/
a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null
This rule optionally takes a second argument, which should be an object with the following supported properties:
-
"null"
: Customize how this rule treatsnull
literals. Possible values:-
always
(default) - Always use === or !==. -
never
- Never use === or !== withnull
. -
ignore
- Do not apply this rule tonull
.
-
smart
The "smart"
option enforces the use of ===
and !==
except for these cases:
- Comparing two literal values
- Evaluating the value of
typeof
- Comparing against
null
Examples of incorrect code for the "smart"
option:
/*eslint eqeqeq: ["error", "smart"]*/
// comparing two variables requires ===
a == b
// only one side is a literal
foo == true
bananas != 1
// comparing to undefined requires ===
value == undefined
Examples of correct code for the "smart"
option:
/*eslint eqeqeq: ["error", "smart"]*/
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null
allow-null
Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell eslint to always enforce strict equality except when comparing with the null
literal.
["error", "always", {"null": "ignore"}]
When Not To Use It
If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/
Don't use IDs in selectors. Open
#footer p {
- Exclude checks
Don't use IDs in selectors. Open
#footer a {
- Exclude checks
Heading (h6) has already been defined. Open
h1, h2, h3, h4, h5, h6 {font-weight:normal;color:#111;}
- Exclude checks
Heading (h3) has already been defined. Open
h3 {font-size:1.5em;line-height:1;margin-bottom:1em;}
- Exclude checks
Using width with padding can sometimes make elements larger than you expect. Open
textarea {width:390px;height:250px;padding:5px;}
- Exclude checks
Rule doesn't have all its properties in alphabetical order. Open
.success {background:#E6EFC2;color:#264409;border-color:#C6D880;}
- Exclude checks
Expected '===' and instead saw '=='. Open
if (request.readyState == 4){
- Read upRead up
- Exclude checks
Require === and !== (eqeqeq)
It is considered good practice to use the type-safe equality operators ===
and !==
instead of their regular counterparts ==
and !=
.
The reason for this is that ==
and !=
do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm.
For instance, the following statements are all considered true
:
[] == false
[] == ![]
3 == "03"
If one of those occurs in an innocent-looking statement such as a == b
the actual problem is very difficult to spot.
Rule Details
This rule is aimed at eliminating the type-unsafe equality operators.
Examples of incorrect code for this rule:
/*eslint eqeqeq: "error"*/
if (x == 42) { }
if ("" == text) { }
if (obj.getStuff() != undefined) { }
The --fix
option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof
expression, or if both operands are literals with the same type.
Options
always
The "always"
option (default) enforces the use of ===
and !==
in every situation (except when you opt-in to more specific handling of null
[see below]).
Examples of incorrect code for the "always"
option:
/*eslint eqeqeq: ["error", "always"]*/
a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null
Examples of correct code for the "always"
option:
/*eslint eqeqeq: ["error", "always"]*/
a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null
This rule optionally takes a second argument, which should be an object with the following supported properties:
-
"null"
: Customize how this rule treatsnull
literals. Possible values:-
always
(default) - Always use === or !==. -
never
- Never use === or !== withnull
. -
ignore
- Do not apply this rule tonull
.
-
smart
The "smart"
option enforces the use of ===
and !==
except for these cases:
- Comparing two literal values
- Evaluating the value of
typeof
- Comparing against
null
Examples of incorrect code for the "smart"
option:
/*eslint eqeqeq: ["error", "smart"]*/
// comparing two variables requires ===
a == b
// only one side is a literal
foo == true
bananas != 1
// comparing to undefined requires ===
value == undefined
Examples of correct code for the "smart"
option:
/*eslint eqeqeq: ["error", "smart"]*/
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null
allow-null
Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell eslint to always enforce strict equality except when comparing with the null
literal.
["error", "always", {"null": "ignore"}]
When Not To Use It
If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/
Unnecessary semicolon. Open
};
- Read upRead up
- Exclude checks
disallow unnecessary semicolons (no-extra-semi)
Typing mistakes and misunderstandings about where semicolons are required can lead to semicolons that are unnecessary. While not technically an error, extra semicolons can cause confusion when reading code.
Rule Details
This rule disallows unnecessary semicolons.
Examples of incorrect code for this rule:
/*eslint no-extra-semi: "error"*/
var x = 5;;
function foo() {
// code
};
Examples of correct code for this rule:
/*eslint no-extra-semi: "error"*/
var x = 5;
var foo = function() {
// code
};
When Not To Use It
If you intentionally use extra semicolons then you can disable this rule.
Related Rules
- [semi](semi.md)
- [semi-spacing](semi-spacing.md) Source: http://eslint.org/docs/rules/
Expected '===' and instead saw '=='. Open
if(evLi.event_type == 'click') {
- Read upRead up
- Exclude checks
Require === and !== (eqeqeq)
It is considered good practice to use the type-safe equality operators ===
and !==
instead of their regular counterparts ==
and !=
.
The reason for this is that ==
and !=
do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm.
For instance, the following statements are all considered true
:
[] == false
[] == ![]
3 == "03"
If one of those occurs in an innocent-looking statement such as a == b
the actual problem is very difficult to spot.
Rule Details
This rule is aimed at eliminating the type-unsafe equality operators.
Examples of incorrect code for this rule:
/*eslint eqeqeq: "error"*/
if (x == 42) { }
if ("" == text) { }
if (obj.getStuff() != undefined) { }
The --fix
option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof
expression, or if both operands are literals with the same type.
Options
always
The "always"
option (default) enforces the use of ===
and !==
in every situation (except when you opt-in to more specific handling of null
[see below]).
Examples of incorrect code for the "always"
option:
/*eslint eqeqeq: ["error", "always"]*/
a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null
Examples of correct code for the "always"
option:
/*eslint eqeqeq: ["error", "always"]*/
a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null
This rule optionally takes a second argument, which should be an object with the following supported properties:
-
"null"
: Customize how this rule treatsnull
literals. Possible values:-
always
(default) - Always use === or !==. -
never
- Never use === or !== withnull
. -
ignore
- Do not apply this rule tonull
.
-
smart
The "smart"
option enforces the use of ===
and !==
except for these cases:
- Comparing two literal values
- Evaluating the value of
typeof
- Comparing against
null
Examples of incorrect code for the "smart"
option:
/*eslint eqeqeq: ["error", "smart"]*/
// comparing two variables requires ===
a == b
// only one side is a literal
foo == true
bananas != 1
// comparing to undefined requires ===
value == undefined
Examples of correct code for the "smart"
option:
/*eslint eqeqeq: ["error", "smart"]*/
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null
allow-null
Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell eslint to always enforce strict equality except when comparing with the null
literal.
["error", "always", {"null": "ignore"}]
When Not To Use It
If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/
Unexpected alert. Open
alert(this.api + ' not supported by Mapstraction.removeEvent');
- Read upRead up
- Exclude checks
Disallow Use of Alert (no-alert)
JavaScript's alert
, confirm
, and prompt
functions are widely considered to be obtrusive as UI elements and should be replaced by a more appropriate custom UI implementation. Furthermore, alert
is often used while debugging code, which should be removed before deployment to production.
alert("here!");
Rule Details
This rule is aimed at catching debugging code that should be removed and popup UI elements that should be replaced with less obtrusive, custom UIs. As such, it will warn when it encounters alert
, prompt
, and confirm
function calls which are not shadowed.
Examples of incorrect code for this rule:
/*eslint no-alert: "error"*/
alert("here!");
confirm("Are you sure?");
prompt("What's your name?", "John Doe");
Examples of correct code for this rule:
/*eslint no-alert: "error"*/
customAlert("Something happened!");
customConfirm("Are you sure?");
customPrompt("Who are you?");
function foo() {
var alert = myCustomLib.customAlert;
alert();
}
Related Rules
- [no-console](no-console.md)
- [no-debugger](no-debugger.md) Source: http://eslint.org/docs/rules/
Expected '===' and instead saw '=='. Open
else if ( args.zoom == 'small' ) {
- Read upRead up
- Exclude checks
Require === and !== (eqeqeq)
It is considered good practice to use the type-safe equality operators ===
and !==
instead of their regular counterparts ==
and !=
.
The reason for this is that ==
and !=
do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm.
For instance, the following statements are all considered true
:
[] == false
[] == ![]
3 == "03"
If one of those occurs in an innocent-looking statement such as a == b
the actual problem is very difficult to spot.
Rule Details
This rule is aimed at eliminating the type-unsafe equality operators.
Examples of incorrect code for this rule:
/*eslint eqeqeq: "error"*/
if (x == 42) { }
if ("" == text) { }
if (obj.getStuff() != undefined) { }
The --fix
option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof
expression, or if both operands are literals with the same type.
Options
always
The "always"
option (default) enforces the use of ===
and !==
in every situation (except when you opt-in to more specific handling of null
[see below]).
Examples of incorrect code for the "always"
option:
/*eslint eqeqeq: ["error", "always"]*/
a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null
Examples of correct code for the "always"
option:
/*eslint eqeqeq: ["error", "always"]*/
a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null
This rule optionally takes a second argument, which should be an object with the following supported properties:
-
"null"
: Customize how this rule treatsnull
literals. Possible values:-
always
(default) - Always use === or !==. -
never
- Never use === or !== withnull
. -
ignore
- Do not apply this rule tonull
.
-
smart
The "smart"
option enforces the use of ===
and !==
except for these cases:
- Comparing two literal values
- Evaluating the value of
typeof
- Comparing against
null
Examples of incorrect code for the "smart"
option:
/*eslint eqeqeq: ["error", "smart"]*/
// comparing two variables requires ===
a == b
// only one side is a literal
foo == true
bananas != 1
// comparing to undefined requires ===
value == undefined
Examples of correct code for the "smart"
option:
/*eslint eqeqeq: ["error", "smart"]*/
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null
allow-null
Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell eslint to always enforce strict equality except when comparing with the null
literal.
["error", "always", {"null": "ignore"}]
When Not To Use It
If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/
Expected '===' and instead saw '=='. Open
else if ( args.zoom == 'small' ) {
- Read upRead up
- Exclude checks
Require === and !== (eqeqeq)
It is considered good practice to use the type-safe equality operators ===
and !==
instead of their regular counterparts ==
and !=
.
The reason for this is that ==
and !=
do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm.
For instance, the following statements are all considered true
:
[] == false
[] == ![]
3 == "03"
If one of those occurs in an innocent-looking statement such as a == b
the actual problem is very difficult to spot.
Rule Details
This rule is aimed at eliminating the type-unsafe equality operators.
Examples of incorrect code for this rule:
/*eslint eqeqeq: "error"*/
if (x == 42) { }
if ("" == text) { }
if (obj.getStuff() != undefined) { }
The --fix
option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof
expression, or if both operands are literals with the same type.
Options
always
The "always"
option (default) enforces the use of ===
and !==
in every situation (except when you opt-in to more specific handling of null
[see below]).
Examples of incorrect code for the "always"
option:
/*eslint eqeqeq: ["error", "always"]*/
a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null
Examples of correct code for the "always"
option:
/*eslint eqeqeq: ["error", "always"]*/
a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null
This rule optionally takes a second argument, which should be an object with the following supported properties:
-
"null"
: Customize how this rule treatsnull
literals. Possible values:-
always
(default) - Always use === or !==. -
never
- Never use === or !== withnull
. -
ignore
- Do not apply this rule tonull
.
-
smart
The "smart"
option enforces the use of ===
and !==
except for these cases:
- Comparing two literal values
- Evaluating the value of
typeof
- Comparing against
null
Examples of incorrect code for the "smart"
option:
/*eslint eqeqeq: ["error", "smart"]*/
// comparing two variables requires ===
a == b
// only one side is a literal
foo == true
bananas != 1
// comparing to undefined requires ===
value == undefined
Examples of correct code for the "smart"
option:
/*eslint eqeqeq: ["error", "smart"]*/
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null
allow-null
Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell eslint to always enforce strict equality except when comparing with the null
literal.
["error", "always", {"null": "ignore"}]
When Not To Use It
If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/
'i' is already defined. Open
for(var i = 0, length = this.polylines.length; i < length; i++){
- Read upRead up
- Exclude checks
disallow variable redeclaration (no-redeclare)
In JavaScript, it's possible to redeclare the same variable name using var
. This can lead to confusion as to where the variable is actually declared and initialized.
Rule Details
This rule is aimed at eliminating variables that have multiple declarations in the same scope.
Examples of incorrect code for this rule:
/*eslint no-redeclare: "error"*/
var a = 3;
var a = 10;
Examples of correct code for this rule:
/*eslint no-redeclare: "error"*/
var a = 3;
// ...
a = 10;
Options
This rule takes one optional argument, an object with a boolean property "builtinGlobals"
. It defaults to false
.
If set to true
, this rule also checks redeclaration of built-in globals, such as Object
, Array
, Number
...
builtinGlobals
Examples of incorrect code for the { "builtinGlobals": true }
option:
/*eslint no-redeclare: ["error", { "builtinGlobals": true }]*/
var Object = 0;
Examples of incorrect code for the { "builtinGlobals": true }
option and the browser
environment:
/*eslint no-redeclare: ["error", { "builtinGlobals": true }]*/
/*eslint-env browser*/
var top = 0;
The browser
environment has many built-in global variables (for example, top
). Some of built-in global variables cannot be redeclared.
Source: http://eslint.org/docs/rules/
Unexpected alert. Open
alert(this.api + ' not supported by Mapstraction.setBounds');
- Read upRead up
- Exclude checks
Disallow Use of Alert (no-alert)
JavaScript's alert
, confirm
, and prompt
functions are widely considered to be obtrusive as UI elements and should be replaced by a more appropriate custom UI implementation. Furthermore, alert
is often used while debugging code, which should be removed before deployment to production.
alert("here!");
Rule Details
This rule is aimed at catching debugging code that should be removed and popup UI elements that should be replaced with less obtrusive, custom UIs. As such, it will warn when it encounters alert
, prompt
, and confirm
function calls which are not shadowed.
Examples of incorrect code for this rule:
/*eslint no-alert: "error"*/
alert("here!");
confirm("Are you sure?");
prompt("What's your name?", "John Doe");
Examples of correct code for this rule:
/*eslint no-alert: "error"*/
customAlert("Something happened!");
customConfirm("Are you sure?");
customPrompt("Who are you?");
function foo() {
var alert = myCustomLib.customAlert;
alert();
}
Related Rules
- [no-console](no-console.md)
- [no-debugger](no-debugger.md) Source: http://eslint.org/docs/rules/
Don't use IDs in selectors. Open
#sidebar p {
- Exclude checks
Rule is empty. Open
#calendar {
- Exclude checks
Don't use IDs in selectors. Open
#calendar tfoot td {
- Exclude checks
Using height with padding can sometimes make elements larger than you expect. Open
textarea {width:390px;height:250px;padding:5px;}
- Exclude checks