Showing 20,971 of 20,976 total issues
Move the invocation into the parens that contain the function. Open
(function ($) {
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Require IIFEs to be Wrapped (wrap-iife)
You can immediately invoke function expressions, but not function declarations. A common technique to create an immediately-invoked function expression (IIFE) is to wrap a function declaration in parentheses. The opening parentheses causes the contained function to be parsed as an expression, rather than a declaration.
// function expression could be unwrapped
var x = function () { return { y: 1 };}();
// function declaration must be wrapped
function () { /* side effects */ }(); // SyntaxError
Rule Details
This rule requires all immediately-invoked function expressions to be wrapped in parentheses.
Options
This rule has two options, a string option and an object option.
String option:
-
"outside"
enforces always wrapping the call expression. The default is"outside"
. -
"inside"
enforces always wrapping the function expression. -
"any"
enforces always wrapping, but allows either style.
Object option:
-
"functionPrototypeMethods": true
additionally enforces wrapping function expressions invoked using.call
and.apply
. The default isfalse
.
outside
Examples of incorrect code for the default "outside"
option:
/*eslint wrap-iife: ["error", "outside"]*/
var x = function () { return { y: 1 };}(); // unwrapped
var x = (function () { return { y: 1 };})(); // wrapped function expression
Examples of correct code for the default "outside"
option:
/*eslint wrap-iife: ["error", "outside"]*/
var x = (function () { return { y: 1 };}()); // wrapped call expression
inside
Examples of incorrect code for the "inside"
option:
/*eslint wrap-iife: ["error", "inside"]*/
var x = function () { return { y: 1 };}(); // unwrapped
var x = (function () { return { y: 1 };}()); // wrapped call expression
Examples of correct code for the "inside"
option:
/*eslint wrap-iife: ["error", "inside"]*/
var x = (function () { return { y: 1 };})(); // wrapped function expression
any
Examples of incorrect code for the "any"
option:
/*eslint wrap-iife: ["error", "any"]*/
var x = function () { return { y: 1 };}(); // unwrapped
Examples of correct code for the "any"
option:
/*eslint wrap-iife: ["error", "any"]*/
var x = (function () { return { y: 1 };}()); // wrapped call expression
var x = (function () { return { y: 1 };})(); // wrapped function expression
functionPrototypeMethods
Examples of incorrect code for this rule with the "inside", { "functionPrototypeMethods": true }
options:
/* eslint wrap-iife: [2, "inside", { functionPrototypeMethods: true }] */
var x = function(){ foo(); }()
var x = (function(){ foo(); }())
var x = function(){ foo(); }.call(bar)
var x = (function(){ foo(); }.call(bar))
Examples of correct code for this rule with the "inside", { "functionPrototypeMethods": true }
options:
/* eslint wrap-iife: [2, "inside", { functionPrototypeMethods: true }] */
var x = (function(){ foo(); })()
var x = (function(){ foo(); }).call(bar)
Source: http://eslint.org/docs/rules/
Expected an assignment or function call and instead saw an expression. Open
self.isOpen ? self.close() : self.open();
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Disallow Unused Expressions (no-unused-expressions)
An unused expression which has no effect on the state of the program indicates a logic error.
For example, n + 1;
is not a syntax error, but it might be a typing mistake where a programmer meant an assignment statement n += 1;
instead.
Rule Details
This rule aims to eliminate unused expressions which have no effect on the state of the program.
This rule does not apply to function calls or constructor calls with the new
operator, because they could have side effects on the state of the program.
var i = 0;
function increment() { i += 1; }
increment(); // return value is unused, but i changed as a side effect
var nThings = 0;
function Thing() { nThings += 1; }
new Thing(); // constructed object is unused, but nThings changed as a side effect
This rule does not apply to directives (which are in the form of literal string expressions such as "use strict";
at the beginning of a script, module, or function).
Sequence expressions (those using a comma, such as a = 1, b = 2
) are always considered unused unless their return value is assigned or used in a condition evaluation, or a function call is made with the sequence expression value.
Options
This rule, in its default state, does not require any arguments. If you would like to enable one or more of the following you may pass an object with the options set as follows:
-
allowShortCircuit
set totrue
will allow you to use short circuit evaluations in your expressions (Default:false
). -
allowTernary
set totrue
will enable you to use ternary operators in your expressions similarly to short circuit evaluations (Default:false
). -
allowTaggedTemplates
set totrue
will enable you to use tagged template literals in your expressions (Default:false
).
These options allow unused expressions only if all of the code paths either directly change the state (for example, assignment statement) or could have side effects (for example, function call).
Examples of incorrect code for the default { "allowShortCircuit": false, "allowTernary": false }
options:
/*eslint no-unused-expressions: "error"*/
0
if(0) 0
{0}
f(0), {}
a && b()
a, b()
c = a, b;
a() && function namedFunctionInExpressionContext () {f();}
(function anIncompleteIIFE () {});
injectGlobal`body{ color: red; }`
Note that one or more string expression statements (with or without semi-colons) will only be considered as unused if they are not in the beginning of a script, module, or function (alone and uninterrupted by other statements). Otherwise, they will be treated as part of a "directive prologue", a section potentially usable by JavaScript engines. This includes "strict mode" directives.
"use strict";
"use asm"
"use stricter";
"use babel"
"any other strings like this in the prologue";
Examples of correct code for the default { "allowShortCircuit": false, "allowTernary": false }
options:
/*eslint no-unused-expressions: "error"*/
{} // In this context, this is a block statement, not an object literal
{myLabel: someVar} // In this context, this is a block statement with a label and expression, not an object literal
function namedFunctionDeclaration () {}
(function aGenuineIIFE () {}());
f()
a = 0
new C
delete a.b
void a
allowShortCircuit
Examples of incorrect code for the { "allowShortCircuit": true }
option:
/*eslint no-unused-expressions: ["error", { "allowShortCircuit": true }]*/
a || b
Examples of correct code for the { "allowShortCircuit": true }
option:
/*eslint no-unused-expressions: ["error", { "allowShortCircuit": true }]*/
a && b()
a() || (b = c)
allowTernary
Examples of incorrect code for the { "allowTernary": true }
option:
/*eslint no-unused-expressions: ["error", { "allowTernary": true }]*/
a ? b : 0
a ? b : c()
Examples of correct code for the { "allowTernary": true }
option:
/*eslint no-unused-expressions: ["error", { "allowTernary": true }]*/
a ? b() : c()
a ? (b = c) : d()
allowShortCircuit and allowTernary
Examples of correct code for the { "allowShortCircuit": true, "allowTernary": true }
options:
/*eslint no-unused-expressions: ["error", { "allowShortCircuit": true, "allowTernary": true }]*/
a ? b() || (c = d) : e()
allowTaggedTemplates
Examples of incorrect code for the { "allowTaggedTemplates": true }
option:
/*eslint no-unused-expressions: ["error", { "allowTaggedTemplates": true }]*/
`some untagged template string`;
Examples of correct code for the { "allowTaggedTemplates": true }
option:
/*eslint no-unused-expressions: ["error", { "allowTaggedTemplates": true }]*/
tag`some tagged template string`;
Source: http://eslint.org/docs/rules/
Move the invocation into the parens that contain the function. Open
self.unlock = (function() {
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- Exclude checks
Require IIFEs to be Wrapped (wrap-iife)
You can immediately invoke function expressions, but not function declarations. A common technique to create an immediately-invoked function expression (IIFE) is to wrap a function declaration in parentheses. The opening parentheses causes the contained function to be parsed as an expression, rather than a declaration.
// function expression could be unwrapped
var x = function () { return { y: 1 };}();
// function declaration must be wrapped
function () { /* side effects */ }(); // SyntaxError
Rule Details
This rule requires all immediately-invoked function expressions to be wrapped in parentheses.
Options
This rule has two options, a string option and an object option.
String option:
-
"outside"
enforces always wrapping the call expression. The default is"outside"
. -
"inside"
enforces always wrapping the function expression. -
"any"
enforces always wrapping, but allows either style.
Object option:
-
"functionPrototypeMethods": true
additionally enforces wrapping function expressions invoked using.call
and.apply
. The default isfalse
.
outside
Examples of incorrect code for the default "outside"
option:
/*eslint wrap-iife: ["error", "outside"]*/
var x = function () { return { y: 1 };}(); // unwrapped
var x = (function () { return { y: 1 };})(); // wrapped function expression
Examples of correct code for the default "outside"
option:
/*eslint wrap-iife: ["error", "outside"]*/
var x = (function () { return { y: 1 };}()); // wrapped call expression
inside
Examples of incorrect code for the "inside"
option:
/*eslint wrap-iife: ["error", "inside"]*/
var x = function () { return { y: 1 };}(); // unwrapped
var x = (function () { return { y: 1 };}()); // wrapped call expression
Examples of correct code for the "inside"
option:
/*eslint wrap-iife: ["error", "inside"]*/
var x = (function () { return { y: 1 };})(); // wrapped function expression
any
Examples of incorrect code for the "any"
option:
/*eslint wrap-iife: ["error", "any"]*/
var x = function () { return { y: 1 };}(); // unwrapped
Examples of correct code for the "any"
option:
/*eslint wrap-iife: ["error", "any"]*/
var x = (function () { return { y: 1 };}()); // wrapped call expression
var x = (function () { return { y: 1 };})(); // wrapped function expression
functionPrototypeMethods
Examples of incorrect code for this rule with the "inside", { "functionPrototypeMethods": true }
options:
/* eslint wrap-iife: [2, "inside", { functionPrototypeMethods: true }] */
var x = function(){ foo(); }()
var x = (function(){ foo(); }())
var x = function(){ foo(); }.call(bar)
var x = (function(){ foo(); }.call(bar))
Examples of correct code for this rule with the "inside", { "functionPrototypeMethods": true }
options:
/* eslint wrap-iife: [2, "inside", { functionPrototypeMethods: true }] */
var x = (function(){ foo(); })()
var x = (function(){ foo(); }).call(bar)
Source: http://eslint.org/docs/rules/
Unexpected prompt. Open
url = window.prompt(dialogReplacements.url, a.href);
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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/
Move the invocation into the parens that contain the function. Open
this.onKeyDown = (function() {
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- Exclude checks
Require IIFEs to be Wrapped (wrap-iife)
You can immediately invoke function expressions, but not function declarations. A common technique to create an immediately-invoked function expression (IIFE) is to wrap a function declaration in parentheses. The opening parentheses causes the contained function to be parsed as an expression, rather than a declaration.
// function expression could be unwrapped
var x = function () { return { y: 1 };}();
// function declaration must be wrapped
function () { /* side effects */ }(); // SyntaxError
Rule Details
This rule requires all immediately-invoked function expressions to be wrapped in parentheses.
Options
This rule has two options, a string option and an object option.
String option:
-
"outside"
enforces always wrapping the call expression. The default is"outside"
. -
"inside"
enforces always wrapping the function expression. -
"any"
enforces always wrapping, but allows either style.
Object option:
-
"functionPrototypeMethods": true
additionally enforces wrapping function expressions invoked using.call
and.apply
. The default isfalse
.
outside
Examples of incorrect code for the default "outside"
option:
/*eslint wrap-iife: ["error", "outside"]*/
var x = function () { return { y: 1 };}(); // unwrapped
var x = (function () { return { y: 1 };})(); // wrapped function expression
Examples of correct code for the default "outside"
option:
/*eslint wrap-iife: ["error", "outside"]*/
var x = (function () { return { y: 1 };}()); // wrapped call expression
inside
Examples of incorrect code for the "inside"
option:
/*eslint wrap-iife: ["error", "inside"]*/
var x = function () { return { y: 1 };}(); // unwrapped
var x = (function () { return { y: 1 };}()); // wrapped call expression
Examples of correct code for the "inside"
option:
/*eslint wrap-iife: ["error", "inside"]*/
var x = (function () { return { y: 1 };})(); // wrapped function expression
any
Examples of incorrect code for the "any"
option:
/*eslint wrap-iife: ["error", "any"]*/
var x = function () { return { y: 1 };}(); // unwrapped
Examples of correct code for the "any"
option:
/*eslint wrap-iife: ["error", "any"]*/
var x = (function () { return { y: 1 };}()); // wrapped call expression
var x = (function () { return { y: 1 };})(); // wrapped function expression
functionPrototypeMethods
Examples of incorrect code for this rule with the "inside", { "functionPrototypeMethods": true }
options:
/* eslint wrap-iife: [2, "inside", { functionPrototypeMethods: true }] */
var x = function(){ foo(); }()
var x = (function(){ foo(); }())
var x = function(){ foo(); }.call(bar)
var x = (function(){ foo(); }.call(bar))
Examples of correct code for this rule with the "inside", { "functionPrototypeMethods": true }
options:
/* eslint wrap-iife: [2, "inside", { functionPrototypeMethods: true }] */
var x = (function(){ foo(); })()
var x = (function(){ foo(); }).call(bar)
Source: http://eslint.org/docs/rules/
The body of a for-in should be wrapped in an if statement to filter unwanted properties from the prototype. Open
for (tagName in this.options.rules.msWordMarkup.tags) {
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Require Guarding for-in (guard-for-in)
Looping over objects with a for in
loop will include properties that are inherited through the prototype chain. This behavior can lead to unexpected items in your for loop.
for (key in foo) {
doSomething(key);
}
Note that simply checking foo.hasOwnProperty(key)
is likely to cause an error in some cases; see [no-prototype-builtins](no-prototype-builtins.md).
Rule Details
This rule is aimed at preventing unexpected behavior that could arise from using a for in
loop without filtering the results in the loop. As such, it will warn when for in
loops do not filter their results with an if
statement.
Examples of incorrect code for this rule:
/*eslint guard-for-in: "error"*/
for (key in foo) {
doSomething(key);
}
Examples of correct code for this rule:
/*eslint guard-for-in: "error"*/
for (key in foo) {
if (Object.prototype.hasOwnProperty.call(foo, key)) {
doSomething(key);
}
if ({}.hasOwnProperty.call(foo, key)) {
doSomething(key);
}
}
Related Rules
- [no-prototype-builtins](no-prototype-builtins.md)
Further Reading
Move the invocation into the parens that contain the function. Open
var asciifold = (function() {
- Read upRead up
- Exclude checks
Require IIFEs to be Wrapped (wrap-iife)
You can immediately invoke function expressions, but not function declarations. A common technique to create an immediately-invoked function expression (IIFE) is to wrap a function declaration in parentheses. The opening parentheses causes the contained function to be parsed as an expression, rather than a declaration.
// function expression could be unwrapped
var x = function () { return { y: 1 };}();
// function declaration must be wrapped
function () { /* side effects */ }(); // SyntaxError
Rule Details
This rule requires all immediately-invoked function expressions to be wrapped in parentheses.
Options
This rule has two options, a string option and an object option.
String option:
-
"outside"
enforces always wrapping the call expression. The default is"outside"
. -
"inside"
enforces always wrapping the function expression. -
"any"
enforces always wrapping, but allows either style.
Object option:
-
"functionPrototypeMethods": true
additionally enforces wrapping function expressions invoked using.call
and.apply
. The default isfalse
.
outside
Examples of incorrect code for the default "outside"
option:
/*eslint wrap-iife: ["error", "outside"]*/
var x = function () { return { y: 1 };}(); // unwrapped
var x = (function () { return { y: 1 };})(); // wrapped function expression
Examples of correct code for the default "outside"
option:
/*eslint wrap-iife: ["error", "outside"]*/
var x = (function () { return { y: 1 };}()); // wrapped call expression
inside
Examples of incorrect code for the "inside"
option:
/*eslint wrap-iife: ["error", "inside"]*/
var x = function () { return { y: 1 };}(); // unwrapped
var x = (function () { return { y: 1 };}()); // wrapped call expression
Examples of correct code for the "inside"
option:
/*eslint wrap-iife: ["error", "inside"]*/
var x = (function () { return { y: 1 };})(); // wrapped function expression
any
Examples of incorrect code for the "any"
option:
/*eslint wrap-iife: ["error", "any"]*/
var x = function () { return { y: 1 };}(); // unwrapped
Examples of correct code for the "any"
option:
/*eslint wrap-iife: ["error", "any"]*/
var x = (function () { return { y: 1 };}()); // wrapped call expression
var x = (function () { return { y: 1 };})(); // wrapped function expression
functionPrototypeMethods
Examples of incorrect code for this rule with the "inside", { "functionPrototypeMethods": true }
options:
/* eslint wrap-iife: [2, "inside", { functionPrototypeMethods: true }] */
var x = function(){ foo(); }()
var x = (function(){ foo(); }())
var x = function(){ foo(); }.call(bar)
var x = (function(){ foo(); }.call(bar))
Examples of correct code for this rule with the "inside", { "functionPrototypeMethods": true }
options:
/* eslint wrap-iife: [2, "inside", { functionPrototypeMethods: true }] */
var x = (function(){ foo(); })()
var x = (function(){ foo(); }).call(bar)
Source: http://eslint.org/docs/rules/
Unexpected require(). Open
module.exports = factory(require('jquery'), require('sifter'), require('microplugin'));
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Enforce require() on the top-level module scope (global-require)
In Node.js, module dependencies are included using the require()
function, such as:
var fs = require("fs");
While require()
may be called anywhere in code, some style guides prescribe that it should be called only in the top level of a module to make it easier to identify dependencies. For instance, it's arguably harder to identify dependencies when they are deeply nested inside of functions and other statements:
function foo() {
if (condition) {
var fs = require("fs");
}
}
Since require()
does a synchronous load, it can cause performance problems when used in other locations.
Further, ES6 modules mandate that import
and export
statements can only occur in the top level of the module's body.
Rule Details
This rule requires all calls to require()
to be at the top level of the module, similar to ES6 import
and export
statements, which also can occur only at the top level.
Examples of incorrect code for this rule:
/*eslint global-require: "error"*/
/*eslint-env es6*/
// calling require() inside of a function is not allowed
function readFile(filename, callback) {
var fs = require('fs');
fs.readFile(filename, callback)
}
// conditional requires like this are also not allowed
if (DEBUG) { require('debug'); }
// a require() in a switch statement is also flagged
switch(x) { case '1': require('1'); break; }
// you may not require() inside an arrow function body
var getModule = (name) => require(name);
// you may not require() inside of a function body as well
function getModule(name) { return require(name); }
// you may not require() inside of a try/catch block
try {
require(unsafeModule);
} catch(e) {
console.log(e);
}
Examples of correct code for this rule:
/*eslint global-require: "error"*/
// all these variations of require() are ok
require('x');
var y = require('y');
var z;
z = require('z').initialize();
// requiring a module and using it in a function is ok
var fs = require('fs');
function readFile(filename, callback) {
fs.readFile(filename, callback)
}
// you can use a ternary to determine which module to require
var logger = DEBUG ? require('dev-logger') : require('logger');
// if you want you can require() at the end of your module
function doSomethingA() {}
function doSomethingB() {}
var x = require("x"),
z = require("z");
When Not To Use It
If you have a module that must be initialized with information that comes from the file-system or if a module is only used in very rare situations and will cause significant overhead to load it may make sense to disable the rule. If you need to require()
an optional dependency inside of a try
/catch
, you can disable this rule for just that dependency using the // eslint-disable-line global-require
comment.
Source: http://eslint.org/docs/rules/
Expected a conditional expression and instead saw an assignment. Open
if (value = self.registerOption(data)) {
- Read upRead up
- Exclude checks
disallow assignment operators in conditional statements (no-cond-assign)
In conditional statements, it is very easy to mistype a comparison operator (such as ==
) as an assignment operator (such as =
). For example:
// Check the user's job title
if (user.jobTitle = "manager") {
// user.jobTitle is now incorrect
}
There are valid reasons to use assignment operators in conditional statements. However, it can be difficult to tell whether a specific assignment was intentional.
Rule Details
This rule disallows ambiguous assignment operators in test conditions of if
, for
, while
, and do...while
statements.
Options
This rule has a string option:
-
"except-parens"
(default) allows assignments in test conditions only if they are enclosed in parentheses (for example, to allow reassigning a variable in the test of awhile
ordo...while
loop) -
"always"
disallows all assignments in test conditions
except-parens
Examples of incorrect code for this rule with the default "except-parens"
option:
/*eslint no-cond-assign: "error"*/
// Unintentional assignment
var x;
if (x = 0) {
var b = 1;
}
// Practical example that is similar to an error
function setHeight(someNode) {
"use strict";
do {
someNode.height = "100px";
} while (someNode = someNode.parentNode);
}
Examples of correct code for this rule with the default "except-parens"
option:
/*eslint no-cond-assign: "error"*/
// Assignment replaced by comparison
var x;
if (x === 0) {
var b = 1;
}
// Practical example that wraps the assignment in parentheses
function setHeight(someNode) {
"use strict";
do {
someNode.height = "100px";
} while ((someNode = someNode.parentNode));
}
// Practical example that wraps the assignment and tests for 'null'
function setHeight(someNode) {
"use strict";
do {
someNode.height = "100px";
} while ((someNode = someNode.parentNode) !== null);
}
always
Examples of incorrect code for this rule with the "always"
option:
/*eslint no-cond-assign: ["error", "always"]*/
// Unintentional assignment
var x;
if (x = 0) {
var b = 1;
}
// Practical example that is similar to an error
function setHeight(someNode) {
"use strict";
do {
someNode.height = "100px";
} while (someNode = someNode.parentNode);
}
// Practical example that wraps the assignment in parentheses
function setHeight(someNode) {
"use strict";
do {
someNode.height = "100px";
} while ((someNode = someNode.parentNode));
}
// Practical example that wraps the assignment and tests for 'null'
function setHeight(someNode) {
"use strict";
do {
someNode.height = "100px";
} while ((someNode = someNode.parentNode) !== null);
}
Examples of correct code for this rule with the "always"
option:
/*eslint no-cond-assign: ["error", "always"]*/
// Assignment replaced by comparison
var x;
if (x === 0) {
var b = 1;
}
Related Rules
- [no-extra-parens](no-extra-parens.md) Source: http://eslint.org/docs/rules/
Expected a conditional expression and instead saw an assignment. Open
if (id = this.registerOptionGroup(data)) {
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- Exclude checks
disallow assignment operators in conditional statements (no-cond-assign)
In conditional statements, it is very easy to mistype a comparison operator (such as ==
) as an assignment operator (such as =
). For example:
// Check the user's job title
if (user.jobTitle = "manager") {
// user.jobTitle is now incorrect
}
There are valid reasons to use assignment operators in conditional statements. However, it can be difficult to tell whether a specific assignment was intentional.
Rule Details
This rule disallows ambiguous assignment operators in test conditions of if
, for
, while
, and do...while
statements.
Options
This rule has a string option:
-
"except-parens"
(default) allows assignments in test conditions only if they are enclosed in parentheses (for example, to allow reassigning a variable in the test of awhile
ordo...while
loop) -
"always"
disallows all assignments in test conditions
except-parens
Examples of incorrect code for this rule with the default "except-parens"
option:
/*eslint no-cond-assign: "error"*/
// Unintentional assignment
var x;
if (x = 0) {
var b = 1;
}
// Practical example that is similar to an error
function setHeight(someNode) {
"use strict";
do {
someNode.height = "100px";
} while (someNode = someNode.parentNode);
}
Examples of correct code for this rule with the default "except-parens"
option:
/*eslint no-cond-assign: "error"*/
// Assignment replaced by comparison
var x;
if (x === 0) {
var b = 1;
}
// Practical example that wraps the assignment in parentheses
function setHeight(someNode) {
"use strict";
do {
someNode.height = "100px";
} while ((someNode = someNode.parentNode));
}
// Practical example that wraps the assignment and tests for 'null'
function setHeight(someNode) {
"use strict";
do {
someNode.height = "100px";
} while ((someNode = someNode.parentNode) !== null);
}
always
Examples of incorrect code for this rule with the "always"
option:
/*eslint no-cond-assign: ["error", "always"]*/
// Unintentional assignment
var x;
if (x = 0) {
var b = 1;
}
// Practical example that is similar to an error
function setHeight(someNode) {
"use strict";
do {
someNode.height = "100px";
} while (someNode = someNode.parentNode);
}
// Practical example that wraps the assignment in parentheses
function setHeight(someNode) {
"use strict";
do {
someNode.height = "100px";
} while ((someNode = someNode.parentNode));
}
// Practical example that wraps the assignment and tests for 'null'
function setHeight(someNode) {
"use strict";
do {
someNode.height = "100px";
} while ((someNode = someNode.parentNode) !== null);
}
Examples of correct code for this rule with the "always"
option:
/*eslint no-cond-assign: ["error", "always"]*/
// Assignment replaced by comparison
var x;
if (x === 0) {
var b = 1;
}
Related Rules
- [no-extra-parens](no-extra-parens.md) Source: http://eslint.org/docs/rules/
Move the invocation into the parens that contain the function. Open
this.setup = (function() {
- Read upRead up
- Exclude checks
Require IIFEs to be Wrapped (wrap-iife)
You can immediately invoke function expressions, but not function declarations. A common technique to create an immediately-invoked function expression (IIFE) is to wrap a function declaration in parentheses. The opening parentheses causes the contained function to be parsed as an expression, rather than a declaration.
// function expression could be unwrapped
var x = function () { return { y: 1 };}();
// function declaration must be wrapped
function () { /* side effects */ }(); // SyntaxError
Rule Details
This rule requires all immediately-invoked function expressions to be wrapped in parentheses.
Options
This rule has two options, a string option and an object option.
String option:
-
"outside"
enforces always wrapping the call expression. The default is"outside"
. -
"inside"
enforces always wrapping the function expression. -
"any"
enforces always wrapping, but allows either style.
Object option:
-
"functionPrototypeMethods": true
additionally enforces wrapping function expressions invoked using.call
and.apply
. The default isfalse
.
outside
Examples of incorrect code for the default "outside"
option:
/*eslint wrap-iife: ["error", "outside"]*/
var x = function () { return { y: 1 };}(); // unwrapped
var x = (function () { return { y: 1 };})(); // wrapped function expression
Examples of correct code for the default "outside"
option:
/*eslint wrap-iife: ["error", "outside"]*/
var x = (function () { return { y: 1 };}()); // wrapped call expression
inside
Examples of incorrect code for the "inside"
option:
/*eslint wrap-iife: ["error", "inside"]*/
var x = function () { return { y: 1 };}(); // unwrapped
var x = (function () { return { y: 1 };}()); // wrapped call expression
Examples of correct code for the "inside"
option:
/*eslint wrap-iife: ["error", "inside"]*/
var x = (function () { return { y: 1 };})(); // wrapped function expression
any
Examples of incorrect code for the "any"
option:
/*eslint wrap-iife: ["error", "any"]*/
var x = function () { return { y: 1 };}(); // unwrapped
Examples of correct code for the "any"
option:
/*eslint wrap-iife: ["error", "any"]*/
var x = (function () { return { y: 1 };}()); // wrapped call expression
var x = (function () { return { y: 1 };})(); // wrapped function expression
functionPrototypeMethods
Examples of incorrect code for this rule with the "inside", { "functionPrototypeMethods": true }
options:
/* eslint wrap-iife: [2, "inside", { functionPrototypeMethods: true }] */
var x = function(){ foo(); }()
var x = (function(){ foo(); }())
var x = function(){ foo(); }.call(bar)
var x = (function(){ foo(); }.call(bar))
Examples of correct code for this rule with the "inside", { "functionPrototypeMethods": true }
options:
/* eslint wrap-iife: [2, "inside", { functionPrototypeMethods: true }] */
var x = (function(){ foo(); })()
var x = (function(){ foo(); }).call(bar)
Source: http://eslint.org/docs/rules/
Move the invocation into the parens that contain the function. Open
this.onKeyDown = (function() {
- Read upRead up
- Exclude checks
Require IIFEs to be Wrapped (wrap-iife)
You can immediately invoke function expressions, but not function declarations. A common technique to create an immediately-invoked function expression (IIFE) is to wrap a function declaration in parentheses. The opening parentheses causes the contained function to be parsed as an expression, rather than a declaration.
// function expression could be unwrapped
var x = function () { return { y: 1 };}();
// function declaration must be wrapped
function () { /* side effects */ }(); // SyntaxError
Rule Details
This rule requires all immediately-invoked function expressions to be wrapped in parentheses.
Options
This rule has two options, a string option and an object option.
String option:
-
"outside"
enforces always wrapping the call expression. The default is"outside"
. -
"inside"
enforces always wrapping the function expression. -
"any"
enforces always wrapping, but allows either style.
Object option:
-
"functionPrototypeMethods": true
additionally enforces wrapping function expressions invoked using.call
and.apply
. The default isfalse
.
outside
Examples of incorrect code for the default "outside"
option:
/*eslint wrap-iife: ["error", "outside"]*/
var x = function () { return { y: 1 };}(); // unwrapped
var x = (function () { return { y: 1 };})(); // wrapped function expression
Examples of correct code for the default "outside"
option:
/*eslint wrap-iife: ["error", "outside"]*/
var x = (function () { return { y: 1 };}()); // wrapped call expression
inside
Examples of incorrect code for the "inside"
option:
/*eslint wrap-iife: ["error", "inside"]*/
var x = function () { return { y: 1 };}(); // unwrapped
var x = (function () { return { y: 1 };}()); // wrapped call expression
Examples of correct code for the "inside"
option:
/*eslint wrap-iife: ["error", "inside"]*/
var x = (function () { return { y: 1 };})(); // wrapped function expression
any
Examples of incorrect code for the "any"
option:
/*eslint wrap-iife: ["error", "any"]*/
var x = function () { return { y: 1 };}(); // unwrapped
Examples of correct code for the "any"
option:
/*eslint wrap-iife: ["error", "any"]*/
var x = (function () { return { y: 1 };}()); // wrapped call expression
var x = (function () { return { y: 1 };})(); // wrapped function expression
functionPrototypeMethods
Examples of incorrect code for this rule with the "inside", { "functionPrototypeMethods": true }
options:
/* eslint wrap-iife: [2, "inside", { functionPrototypeMethods: true }] */
var x = function(){ foo(); }()
var x = (function(){ foo(); }())
var x = function(){ foo(); }.call(bar)
var x = (function(){ foo(); }.call(bar))
Examples of correct code for this rule with the "inside", { "functionPrototypeMethods": true }
options:
/* eslint wrap-iife: [2, "inside", { functionPrototypeMethods: true }] */
var x = (function(){ foo(); })()
var x = (function(){ foo(); }).call(bar)
Source: http://eslint.org/docs/rules/
The body of a for-in should be wrapped in an if statement to filter unwanted properties from the prototype. Open
for ( i in set ) {
- Read upRead up
- Exclude checks
Require Guarding for-in (guard-for-in)
Looping over objects with a for in
loop will include properties that are inherited through the prototype chain. This behavior can lead to unexpected items in your for loop.
for (key in foo) {
doSomething(key);
}
Note that simply checking foo.hasOwnProperty(key)
is likely to cause an error in some cases; see [no-prototype-builtins](no-prototype-builtins.md).
Rule Details
This rule is aimed at preventing unexpected behavior that could arise from using a for in
loop without filtering the results in the loop. As such, it will warn when for in
loops do not filter their results with an if
statement.
Examples of incorrect code for this rule:
/*eslint guard-for-in: "error"*/
for (key in foo) {
doSomething(key);
}
Examples of correct code for this rule:
/*eslint guard-for-in: "error"*/
for (key in foo) {
if (Object.prototype.hasOwnProperty.call(foo, key)) {
doSomething(key);
}
if ({}.hasOwnProperty.call(foo, key)) {
doSomething(key);
}
}
Related Rules
- [no-prototype-builtins](no-prototype-builtins.md)
Further Reading
Expected an assignment or function call and instead saw an expression. Open
a ? a[0].appendChild(this.placeholder[0]) : i.item[0].parentNode.insertBefore(this.placeholder[0], (this.direction === "down" ? i.item[0] : i.item[0].nextSibling));
- Read upRead up
- Exclude checks
Disallow Unused Expressions (no-unused-expressions)
An unused expression which has no effect on the state of the program indicates a logic error.
For example, n + 1;
is not a syntax error, but it might be a typing mistake where a programmer meant an assignment statement n += 1;
instead.
Rule Details
This rule aims to eliminate unused expressions which have no effect on the state of the program.
This rule does not apply to function calls or constructor calls with the new
operator, because they could have side effects on the state of the program.
var i = 0;
function increment() { i += 1; }
increment(); // return value is unused, but i changed as a side effect
var nThings = 0;
function Thing() { nThings += 1; }
new Thing(); // constructed object is unused, but nThings changed as a side effect
This rule does not apply to directives (which are in the form of literal string expressions such as "use strict";
at the beginning of a script, module, or function).
Sequence expressions (those using a comma, such as a = 1, b = 2
) are always considered unused unless their return value is assigned or used in a condition evaluation, or a function call is made with the sequence expression value.
Options
This rule, in its default state, does not require any arguments. If you would like to enable one or more of the following you may pass an object with the options set as follows:
-
allowShortCircuit
set totrue
will allow you to use short circuit evaluations in your expressions (Default:false
). -
allowTernary
set totrue
will enable you to use ternary operators in your expressions similarly to short circuit evaluations (Default:false
). -
allowTaggedTemplates
set totrue
will enable you to use tagged template literals in your expressions (Default:false
).
These options allow unused expressions only if all of the code paths either directly change the state (for example, assignment statement) or could have side effects (for example, function call).
Examples of incorrect code for the default { "allowShortCircuit": false, "allowTernary": false }
options:
/*eslint no-unused-expressions: "error"*/
0
if(0) 0
{0}
f(0), {}
a && b()
a, b()
c = a, b;
a() && function namedFunctionInExpressionContext () {f();}
(function anIncompleteIIFE () {});
injectGlobal`body{ color: red; }`
Note that one or more string expression statements (with or without semi-colons) will only be considered as unused if they are not in the beginning of a script, module, or function (alone and uninterrupted by other statements). Otherwise, they will be treated as part of a "directive prologue", a section potentially usable by JavaScript engines. This includes "strict mode" directives.
"use strict";
"use asm"
"use stricter";
"use babel"
"any other strings like this in the prologue";
Examples of correct code for the default { "allowShortCircuit": false, "allowTernary": false }
options:
/*eslint no-unused-expressions: "error"*/
{} // In this context, this is a block statement, not an object literal
{myLabel: someVar} // In this context, this is a block statement with a label and expression, not an object literal
function namedFunctionDeclaration () {}
(function aGenuineIIFE () {}());
f()
a = 0
new C
delete a.b
void a
allowShortCircuit
Examples of incorrect code for the { "allowShortCircuit": true }
option:
/*eslint no-unused-expressions: ["error", { "allowShortCircuit": true }]*/
a || b
Examples of correct code for the { "allowShortCircuit": true }
option:
/*eslint no-unused-expressions: ["error", { "allowShortCircuit": true }]*/
a && b()
a() || (b = c)
allowTernary
Examples of incorrect code for the { "allowTernary": true }
option:
/*eslint no-unused-expressions: ["error", { "allowTernary": true }]*/
a ? b : 0
a ? b : c()
Examples of correct code for the { "allowTernary": true }
option:
/*eslint no-unused-expressions: ["error", { "allowTernary": true }]*/
a ? b() : c()
a ? (b = c) : d()
allowShortCircuit and allowTernary
Examples of correct code for the { "allowShortCircuit": true, "allowTernary": true }
options:
/*eslint no-unused-expressions: ["error", { "allowShortCircuit": true, "allowTernary": true }]*/
a ? b() || (c = d) : e()
allowTaggedTemplates
Examples of incorrect code for the { "allowTaggedTemplates": true }
option:
/*eslint no-unused-expressions: ["error", { "allowTaggedTemplates": true }]*/
`some untagged template string`;
Examples of correct code for the { "allowTaggedTemplates": true }
option:
/*eslint no-unused-expressions: ["error", { "allowTaggedTemplates": true }]*/
tag`some tagged template string`;
Source: http://eslint.org/docs/rules/
The body of a for-in should be wrapped in an if statement to filter unwanted properties from the prototype. Open
for (attrName in rules.rmAttr) {
- Read upRead up
- Exclude checks
Require Guarding for-in (guard-for-in)
Looping over objects with a for in
loop will include properties that are inherited through the prototype chain. This behavior can lead to unexpected items in your for loop.
for (key in foo) {
doSomething(key);
}
Note that simply checking foo.hasOwnProperty(key)
is likely to cause an error in some cases; see [no-prototype-builtins](no-prototype-builtins.md).
Rule Details
This rule is aimed at preventing unexpected behavior that could arise from using a for in
loop without filtering the results in the loop. As such, it will warn when for in
loops do not filter their results with an if
statement.
Examples of incorrect code for this rule:
/*eslint guard-for-in: "error"*/
for (key in foo) {
doSomething(key);
}
Examples of correct code for this rule:
/*eslint guard-for-in: "error"*/
for (key in foo) {
if (Object.prototype.hasOwnProperty.call(foo, key)) {
doSomething(key);
}
if ({}.hasOwnProperty.call(foo, key)) {
doSomething(key);
}
}
Related Rules
- [no-prototype-builtins](no-prototype-builtins.md)
Further Reading
Unexpected prompt. Open
url = window.prompt("URL", a.href);
- 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/
unnecessary '.apply()'. Open
resize : function() { self.positionDropdown.apply(self, []); },
- Read upRead up
- Exclude checks
Disallow unnecessary .call()
and .apply()
. (no-useless-call)
The function invocation can be written by Function.prototype.call()
and Function.prototype.apply()
.
But Function.prototype.call()
and Function.prototype.apply()
are slower than the normal function invocation.
Rule Details
This rule is aimed to flag usage of Function.prototype.call()
and Function.prototype.apply()
that can be replaced with the normal function invocation.
Examples of incorrect code for this rule:
/*eslint no-useless-call: "error"*/
// These are same as `foo(1, 2, 3);`
foo.call(undefined, 1, 2, 3);
foo.apply(undefined, [1, 2, 3]);
foo.call(null, 1, 2, 3);
foo.apply(null, [1, 2, 3]);
// These are same as `obj.foo(1, 2, 3);`
obj.foo.call(obj, 1, 2, 3);
obj.foo.apply(obj, [1, 2, 3]);
Examples of correct code for this rule:
/*eslint no-useless-call: "error"*/
// The `this` binding is different.
foo.call(obj, 1, 2, 3);
foo.apply(obj, [1, 2, 3]);
obj.foo.call(null, 1, 2, 3);
obj.foo.apply(null, [1, 2, 3]);
obj.foo.call(otherObj, 1, 2, 3);
obj.foo.apply(otherObj, [1, 2, 3]);
// The argument list is variadic.
foo.apply(undefined, args);
foo.apply(null, args);
obj.foo.apply(obj, args);
Known Limitations
This rule compares code statically to check whether or not thisArg
is changed.
So if the code about thisArg
is a dynamic expression, this rule cannot judge correctly.
Examples of incorrect code for this rule:
/*eslint no-useless-call: "error"*/
a[i++].foo.call(a[i++], 1, 2, 3);
Examples of correct code for this rule:
/*eslint no-useless-call: "error"*/
a[++i].foo.call(a[i], 1, 2, 3);
When Not To Use It
If you don't want to be notified about unnecessary .call()
and .apply()
, you can safely disable this rule.
Source: http://eslint.org/docs/rules/
Expected a 'break' statement before 'case'. Open
case KEY_UP:
- Read upRead up
- Exclude checks
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/
Move the invocation into the parens that contain the function. Open
self.setup = (function() {
- Read upRead up
- Exclude checks
Require IIFEs to be Wrapped (wrap-iife)
You can immediately invoke function expressions, but not function declarations. A common technique to create an immediately-invoked function expression (IIFE) is to wrap a function declaration in parentheses. The opening parentheses causes the contained function to be parsed as an expression, rather than a declaration.
// function expression could be unwrapped
var x = function () { return { y: 1 };}();
// function declaration must be wrapped
function () { /* side effects */ }(); // SyntaxError
Rule Details
This rule requires all immediately-invoked function expressions to be wrapped in parentheses.
Options
This rule has two options, a string option and an object option.
String option:
-
"outside"
enforces always wrapping the call expression. The default is"outside"
. -
"inside"
enforces always wrapping the function expression. -
"any"
enforces always wrapping, but allows either style.
Object option:
-
"functionPrototypeMethods": true
additionally enforces wrapping function expressions invoked using.call
and.apply
. The default isfalse
.
outside
Examples of incorrect code for the default "outside"
option:
/*eslint wrap-iife: ["error", "outside"]*/
var x = function () { return { y: 1 };}(); // unwrapped
var x = (function () { return { y: 1 };})(); // wrapped function expression
Examples of correct code for the default "outside"
option:
/*eslint wrap-iife: ["error", "outside"]*/
var x = (function () { return { y: 1 };}()); // wrapped call expression
inside
Examples of incorrect code for the "inside"
option:
/*eslint wrap-iife: ["error", "inside"]*/
var x = function () { return { y: 1 };}(); // unwrapped
var x = (function () { return { y: 1 };}()); // wrapped call expression
Examples of correct code for the "inside"
option:
/*eslint wrap-iife: ["error", "inside"]*/
var x = (function () { return { y: 1 };})(); // wrapped function expression
any
Examples of incorrect code for the "any"
option:
/*eslint wrap-iife: ["error", "any"]*/
var x = function () { return { y: 1 };}(); // unwrapped
Examples of correct code for the "any"
option:
/*eslint wrap-iife: ["error", "any"]*/
var x = (function () { return { y: 1 };}()); // wrapped call expression
var x = (function () { return { y: 1 };})(); // wrapped function expression
functionPrototypeMethods
Examples of incorrect code for this rule with the "inside", { "functionPrototypeMethods": true }
options:
/* eslint wrap-iife: [2, "inside", { functionPrototypeMethods: true }] */
var x = function(){ foo(); }()
var x = (function(){ foo(); }())
var x = function(){ foo(); }.call(bar)
var x = (function(){ foo(); }.call(bar))
Examples of correct code for this rule with the "inside", { "functionPrototypeMethods": true }
options:
/* eslint wrap-iife: [2, "inside", { functionPrototypeMethods: true }] */
var x = (function(){ foo(); })()
var x = (function(){ foo(); }).call(bar)
Source: http://eslint.org/docs/rules/
Expected an assignment or function call and instead saw an expression. Open
(this._mouseStarted && this._mouseUp(event));
- Read upRead up
- Exclude checks
Disallow Unused Expressions (no-unused-expressions)
An unused expression which has no effect on the state of the program indicates a logic error.
For example, n + 1;
is not a syntax error, but it might be a typing mistake where a programmer meant an assignment statement n += 1;
instead.
Rule Details
This rule aims to eliminate unused expressions which have no effect on the state of the program.
This rule does not apply to function calls or constructor calls with the new
operator, because they could have side effects on the state of the program.
var i = 0;
function increment() { i += 1; }
increment(); // return value is unused, but i changed as a side effect
var nThings = 0;
function Thing() { nThings += 1; }
new Thing(); // constructed object is unused, but nThings changed as a side effect
This rule does not apply to directives (which are in the form of literal string expressions such as "use strict";
at the beginning of a script, module, or function).
Sequence expressions (those using a comma, such as a = 1, b = 2
) are always considered unused unless their return value is assigned or used in a condition evaluation, or a function call is made with the sequence expression value.
Options
This rule, in its default state, does not require any arguments. If you would like to enable one or more of the following you may pass an object with the options set as follows:
-
allowShortCircuit
set totrue
will allow you to use short circuit evaluations in your expressions (Default:false
). -
allowTernary
set totrue
will enable you to use ternary operators in your expressions similarly to short circuit evaluations (Default:false
). -
allowTaggedTemplates
set totrue
will enable you to use tagged template literals in your expressions (Default:false
).
These options allow unused expressions only if all of the code paths either directly change the state (for example, assignment statement) or could have side effects (for example, function call).
Examples of incorrect code for the default { "allowShortCircuit": false, "allowTernary": false }
options:
/*eslint no-unused-expressions: "error"*/
0
if(0) 0
{0}
f(0), {}
a && b()
a, b()
c = a, b;
a() && function namedFunctionInExpressionContext () {f();}
(function anIncompleteIIFE () {});
injectGlobal`body{ color: red; }`
Note that one or more string expression statements (with or without semi-colons) will only be considered as unused if they are not in the beginning of a script, module, or function (alone and uninterrupted by other statements). Otherwise, they will be treated as part of a "directive prologue", a section potentially usable by JavaScript engines. This includes "strict mode" directives.
"use strict";
"use asm"
"use stricter";
"use babel"
"any other strings like this in the prologue";
Examples of correct code for the default { "allowShortCircuit": false, "allowTernary": false }
options:
/*eslint no-unused-expressions: "error"*/
{} // In this context, this is a block statement, not an object literal
{myLabel: someVar} // In this context, this is a block statement with a label and expression, not an object literal
function namedFunctionDeclaration () {}
(function aGenuineIIFE () {}());
f()
a = 0
new C
delete a.b
void a
allowShortCircuit
Examples of incorrect code for the { "allowShortCircuit": true }
option:
/*eslint no-unused-expressions: ["error", { "allowShortCircuit": true }]*/
a || b
Examples of correct code for the { "allowShortCircuit": true }
option:
/*eslint no-unused-expressions: ["error", { "allowShortCircuit": true }]*/
a && b()
a() || (b = c)
allowTernary
Examples of incorrect code for the { "allowTernary": true }
option:
/*eslint no-unused-expressions: ["error", { "allowTernary": true }]*/
a ? b : 0
a ? b : c()
Examples of correct code for the { "allowTernary": true }
option:
/*eslint no-unused-expressions: ["error", { "allowTernary": true }]*/
a ? b() : c()
a ? (b = c) : d()
allowShortCircuit and allowTernary
Examples of correct code for the { "allowShortCircuit": true, "allowTernary": true }
options:
/*eslint no-unused-expressions: ["error", { "allowShortCircuit": true, "allowTernary": true }]*/
a ? b() || (c = d) : e()
allowTaggedTemplates
Examples of incorrect code for the { "allowTaggedTemplates": true }
option:
/*eslint no-unused-expressions: ["error", { "allowTaggedTemplates": true }]*/
`some untagged template string`;
Examples of correct code for the { "allowTaggedTemplates": true }
option:
/*eslint no-unused-expressions: ["error", { "allowTaggedTemplates": true }]*/
tag`some tagged template string`;
Source: http://eslint.org/docs/rules/