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Heading (h1) has already been defined. Open
#header, h1, h2, h3, h4 {
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Rule doesn't have all its properties in alphabetical order. Open
code {
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Unexpected alert. Open
alert(address + " found");
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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/
Unexpected alert. Open
alert(api + ' map script not imported');
- 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/
eval can be harmful. Open
eval(' map.addWidget( new ' + pan_zoom_widget + '() );');
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Disallow eval() (no-eval)
JavaScript's eval()
function is potentially dangerous and is often misused. Using eval()
on untrusted code can open a program up to several different injection attacks. The use of eval()
in most contexts can be substituted for a better, alternative approach to a problem.
var obj = { x: "foo" },
key = "x",
value = eval("obj." + key);
Rule Details
This rule is aimed at preventing potentially dangerous, unnecessary, and slow code by disallowing the use of the eval()
function. As such, it will warn whenever the eval()
function is used.
Examples of incorrect code for this rule:
/*eslint no-eval: "error"*/
var obj = { x: "foo" },
key = "x",
value = eval("obj." + key);
(0, eval)("var a = 0");
var foo = eval;
foo("var a = 0");
// This `this` is the global object.
this.eval("var a = 0");
Example of additional incorrect code for this rule when browser
environment is set to true
:
/*eslint no-eval: "error"*/
/*eslint-env browser*/
window.eval("var a = 0");
Example of additional incorrect code for this rule when node
environment is set to true
:
/*eslint no-eval: "error"*/
/*eslint-env node*/
global.eval("var a = 0");
Examples of correct code for this rule:
/*eslint no-eval: "error"*/
/*eslint-env es6*/
var obj = { x: "foo" },
key = "x",
value = obj[key];
class A {
foo() {
// This is a user-defined method.
this.eval("var a = 0");
}
eval() {
}
}
Options
This rule has an option to allow indirect calls to eval
.
Indirect calls to eval
are less dangerous than direct calls to eval
because they cannot dynamically change the scope. Because of this, they also will not negatively impact performance to the degree of direct eval
.
{
"no-eval": ["error", {"allowIndirect": true}] // default is false
}
Example of incorrect code for this rule with the {"allowIndirect": true}
option:
/*eslint no-eval: "error"*/
var obj = { x: "foo" },
key = "x",
value = eval("obj." + key);
Examples of correct code for this rule with the {"allowIndirect": true}
option:
/*eslint no-eval: "error"*/
(0, eval)("var a = 0");
var foo = eval;
foo("var a = 0");
this.eval("var a = 0");
/*eslint no-eval: "error"*/
/*eslint-env browser*/
window.eval("var a = 0");
/*eslint no-eval: "error"*/
/*eslint-env node*/
global.eval("var a = 0");
Known Limitations
- This rule is warning every
eval()
even if theeval
is not global's. This behavior is in order to detect calls of directeval
. Such as:
module.exports = function(eval) {
// If the value of this `eval` is built-in `eval` function, this is a
// call of direct `eval`.
eval("var a = 0");
};
- This rule cannot catch renaming the global object. Such as:
var foo = window;
foo.eval("var a = 0");
Further Reading
Related Rules
- [no-implied-eval](no-implied-eval.md) Source: http://eslint.org/docs/rules/
'i' is already defined. Open
for(var i = 0, length = this.polylines.length; i < length; i++){
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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/
'length' 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.getCenter');
- 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 a 'break' statement before 'case'. Open
case 'openstreetmap':
- Read upRead up
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Disallow Case Statement Fallthrough (no-fallthrough)
The switch
statement in JavaScript is one of the more error-prone constructs of the language thanks in part to the ability to "fall through" from one case
to the next. For example:
switch(foo) {
case 1:
doSomething();
case 2:
doSomethingElse();
}
In this example, if foo
is 1
, then execution will flow through both cases, as the first falls through to the second. You can prevent this by using break
, as in this example:
switch(foo) {
case 1:
doSomething();
break;
case 2:
doSomethingElse();
}
That works fine when you don't want a fallthrough, but what if the fallthrough is intentional, there is no way to indicate that in the language. It's considered a best practice to always indicate when a fallthrough is intentional using a comment which matches the /falls?\s?through/i
regular expression:
switch(foo) {
case 1:
doSomething();
// falls through
case 2:
doSomethingElse();
}
switch(foo) {
case 1:
doSomething();
// fall through
case 2:
doSomethingElse();
}
switch(foo) {
case 1:
doSomething();
// fallsthrough
case 2:
doSomethingElse();
}
In this example, there is no confusion as to the expected behavior. It is clear that the first case is meant to fall through to the second case.
Rule Details
This rule is aimed at eliminating unintentional fallthrough of one case to the other. As such, it flags any fallthrough scenarios that are not marked by a comment.
Examples of incorrect code for this rule:
/*eslint no-fallthrough: "error"*/
switch(foo) {
case 1:
doSomething();
case 2:
doSomething();
}
Examples of correct code for this rule:
/*eslint no-fallthrough: "error"*/
switch(foo) {
case 1:
doSomething();
break;
case 2:
doSomething();
}
function bar(foo) {
switch(foo) {
case 1:
doSomething();
return;
case 2:
doSomething();
}
}
switch(foo) {
case 1:
doSomething();
throw new Error("Boo!");
case 2:
doSomething();
}
switch(foo) {
case 1:
case 2:
doSomething();
}
switch(foo) {
case 1:
doSomething();
// falls through
case 2:
doSomething();
}
Note that the last case
statement in these examples does not cause a warning because there is nothing to fall through into.
Options
This rule accepts a single options argument:
- Set the
commentPattern
option to a regular expression string to change the test for intentional fallthrough comment
commentPattern
Examples of correct code for the { "commentPattern": "break[\\s\\w]*omitted" }
option:
/*eslint no-fallthrough: ["error", { "commentPattern": "break[\\s\\w]*omitted" }]*/
switch(foo) {
case 1:
doSomething();
// break omitted
case 2:
doSomething();
}
switch(foo) {
case 1:
doSomething();
// caution: break is omitted intentionally
default:
doSomething();
}
When Not To Use It
If you don't want to enforce that each case
statement should end with a throw
, return
, break
, or comment, then you can safely turn this rule off.
Related Rules
- [default-case](default-case.md) Source: http://eslint.org/docs/rules/
Unexpected alert. Open
alert(this.api + ' not supported by Mapstraction.getPixelRatio');
- 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/
Rule doesn't have all its properties in alphabetical order. Open
#calendar tfoot td {
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2 IDs in the selector, really? Open
#calendar #today {
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Heading (h4) has already been defined. Open
h4 {font-size:1.2em;line-height:1.25;margin-bottom:1.25em;}
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Rule doesn't have all its properties in alphabetical order. Open
input.text, input.title, textarea, select {margin:0.5em 0;border:1px solid #bbb;}
- Exclude checks
Rule doesn't have all its properties in alphabetical order. Open
.notice {background:#FFF6BF;color:#514721;border-color:#FFD324;}
- Exclude checks
Heading (h2) has already been defined. Open
h2 {
- Exclude checks
Heading (h1) should not be qualified. Open
#header h1 {
- Exclude checks
Values of 0 shouldn't have units specified. Open
top: 0px;
- Exclude checks
'j' is already defined. Open
for(var j=0; j<coordinates.length; j++){
- 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/
Expected '!==' and instead saw '!='. Open
if (this.api != "openstreetmap") {
- 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/