(function next() {

Close

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 is false.

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/

      fs.stat(file, function (err, stat) {

Close

Enforce Callback Error Handling (handle-callback-err)

In Node.js, a common pattern for dealing with asynchronous behavior is called the callback pattern. This pattern expects an Error object or null as the first argument of the callback. Forgetting to handle these errors can lead to some really strange behavior in your application.

function loadData (err, data) {
    doSomething(); // forgot to handle error
}

Rule Details

This rule expects that when you're using the callback pattern in Node.js you'll handle the error.

Options

The rule takes a single string option: the name of the error parameter. The default is "err".

Examples of incorrect code for this rule with the default "err" parameter name:

/*eslint handle-callback-err: "error"*/

function loadData (err, data) {
    doSomething();
}

Examples of correct code for this rule with the default "err" parameter name:

/*eslint handle-callback-err: "error"*/

function loadData (err, data) {
    if (err) {
        console.log(err.stack);
    }
    doSomething();
}

function generateError (err) {
    if (err) {}
}

Examples of correct code for this rule with a sample "error" parameter name:

/*eslint handle-callback-err: ["error", "error"]*/

function loadData (error, data) {
    if (error) {
       console.log(error.stack);
    }
    doSomething();
}

regular expression

Sometimes (especially in big projects) the name of the error variable is not consistent across the project, so you need a more flexible configuration to ensure that the rule reports all unhandled errors.

If the configured name of the error variable begins with a ^ it is considered to be a regexp pattern.

• If the option is "^(err|error|anySpecificError)$", the rule reports unhandled errors where the parameter name can be err, error or anySpecificError.
• If the option is "^.+Error$", the rule reports unhandled errors where the parameter name ends with Error (for example, connectionError or validationError will match).
• If the option is "^.*(e|E)rr", the rule reports unhandled errors where the parameter name matches any string that contains err or Err (for example, err, error, anyError, some_err will match).

When Not To Use It

There are cases where it may be safe for your application to ignore errors, however only ignore errors if you are confident that some other form of monitoring will help you catch the problem.

Further Reading

• The Art Of Node: Callbacks
• Nodejitsu: What are the error conventions? Source: http://eslint.org/docs/rules/

    require([pageModulePath], function (page) {

Close

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/

        ($(this).attr('target') != '_blank') && !$(this).data('go') && !event.ctrlKey && !event.metaKey) {

Close

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 treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

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[r] = i[r] || function () {

Close

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 to true will allow you to use short circuit evaluations in your expressions (Default: false).
• allowTernary set to true will enable you to use ternary operators in your expressions similarly to short circuit evaluations (Default: false).
• allowTaggedTemplates set to true 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/

  isLocalStorageAvailable = (function () {

Close

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 is false.

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/

        require(['highlightjs', 'css!highlightjs-css'], function (highlight) { //load lib on-demand to improve performance

Close

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/

        return (json[this.orderByKey] == option.key) && (json[this.orderDirKey] == option.dir);

Close

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 treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

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/

      if ((value === undefined) || (value === null) || ((typeof value == 'string') && (value.compact() === '')) || value.isDefault) {

Close

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 treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

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/

          walk(file, function (err, res) {

Close

Enforce Callback Error Handling (handle-callback-err)

In Node.js, a common pattern for dealing with asynchronous behavior is called the callback pattern. This pattern expects an Error object or null as the first argument of the callback. Forgetting to handle these errors can lead to some really strange behavior in your application.

function loadData (err, data) {
    doSomething(); // forgot to handle error
}

Rule Details

This rule expects that when you're using the callback pattern in Node.js you'll handle the error.

Options

The rule takes a single string option: the name of the error parameter. The default is "err".

Examples of incorrect code for this rule with the default "err" parameter name:

/*eslint handle-callback-err: "error"*/

function loadData (err, data) {
    doSomething();
}

Examples of correct code for this rule with the default "err" parameter name:

/*eslint handle-callback-err: "error"*/

function loadData (err, data) {
    if (err) {
        console.log(err.stack);
    }
    doSomething();
}

function generateError (err) {
    if (err) {}
}

Examples of correct code for this rule with a sample "error" parameter name:

/*eslint handle-callback-err: ["error", "error"]*/

function loadData (error, data) {
    if (error) {
       console.log(error.stack);
    }
    doSomething();
}

regular expression

Sometimes (especially in big projects) the name of the error variable is not consistent across the project, so you need a more flexible configuration to ensure that the rule reports all unhandled errors.

If the configured name of the error variable begins with a ^ it is considered to be a regexp pattern.

• If the option is "^(err|error|anySpecificError)$", the rule reports unhandled errors where the parameter name can be err, error or anySpecificError.
• If the option is "^.+Error$", the rule reports unhandled errors where the parameter name ends with Error (for example, connectionError or validationError will match).
• If the option is "^.*(e|E)rr", the rule reports unhandled errors where the parameter name matches any string that contains err or Err (for example, err, error, anyError, some_err will match).

When Not To Use It

There are cases where it may be safe for your application to ignore errors, however only ignore errors if you are confident that some other form of monitoring will help you catch the problem.

Further Reading

• The Art Of Node: Callbacks
• Nodejitsu: What are the error conventions? Source: http://eslint.org/docs/rules/

        this.initExtra && this.initExtra.apply(this, Array.prototype.slice.call(arguments, 0));

Close

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 to true will allow you to use short circuit evaluations in your expressions (Default: false).
• allowTernary set to true will enable you to use ternary operators in your expressions similarly to short circuit evaluations (Default: false).
• allowTaggedTemplates set to true 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/

          var _permanent = (permanent || typeof permanent == 'boolean') ? permanent : true;

Close

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 treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

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/

      this.initExtra && this.initExtra.apply(this, Array.prototype.slice.call(arguments, 0));

Close

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 to true will allow you to use short circuit evaluations in your expressions (Default: false).
• allowTernary set to true will enable you to use ternary operators in your expressions similarly to short circuit evaluations (Default: false).
• allowTaggedTemplates set to true 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/

        callback();

Close

Enforce Return After Callback (callback-return)

The callback pattern is at the heart of most I/O and event-driven programming in JavaScript.

function doSomething(err, callback) {
    if (err) {
        return callback(err);
    }
    callback();
}

To prevent calling the callback multiple times it is important to return anytime the callback is triggered outside of the main function body. Neglecting this technique often leads to issues where you do something more than once. For example, in the case of an HTTP request, you may try to send HTTP headers more than once leading Node.js to throw a Can't render headers after they are sent to the client. error.

Rule Details

This rule is aimed at ensuring that callbacks used outside of the main function block are always part-of or immediately preceding a return statement. This rule decides what is a callback based on the name of the function being called.

Options

The rule takes a single option - an array of possible callback names - which may include object methods. The default callback names are callback, cb, next.

Default callback names

Examples of incorrect code for this rule with the default ["callback", "cb", "next"] option:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        callback(err);
    }
    callback();
}

Examples of correct code for this rule with the default ["callback", "cb", "next"] option:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        return callback(err);
    }
    callback();
}

Supplied callback names

Examples of incorrect code for this rule with the option ["done", "send.error", "send.success"]:

/*eslint callback-return: ["error", ["done", "send.error", "send.success"]]*/

function foo(err, done) {
    if (err) {
        done(err);
    }
    done();
}

function bar(err, send) {
    if (err) {
        send.error(err);
    }
    send.success();
}

Examples of correct code for this rule with the option ["done", "send.error", "send.success"]:

/*eslint callback-return: ["error", ["done", "send.error", "send.success"]]*/

function foo(err, done) {
    if (err) {
        return done(err);
    }
    done();
}

function bar(err, send) {
    if (err) {
        return send.error(err);
    }
    send.success();
}

Known Limitations

Because it is difficult to understand the meaning of a program through static analysis, this rule has limitations:

• false negatives when this rule reports correct code, but the program calls the callback more than one time (which is incorrect behavior)
• false positives when this rule reports incorrect code, but the program calls the callback only one time (which is correct behavior)

Passing the callback by reference

The static analysis of this rule does not detect that the program calls the callback if it is an argument of a function (for example, setTimeout).

Example of a false negative when this rule reports correct code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        setTimeout(callback, 0); // this is bad, but WILL NOT warn
    }
    callback();
}

Triggering the callback within a nested function

The static analysis of this rule does not detect that the program calls the callback from within a nested function or an immediately-invoked function expression (IIFE).

Example of a false negative when this rule reports correct code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        process.nextTick(function() {
            return callback(); // this is bad, but WILL NOT warn
        });
    }
    callback();
}

If/else statements

The static analysis of this rule does not detect that the program calls the callback only one time in each branch of an if statement.

Example of a false positive when this rule reports incorrect code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        callback(err); // this is fine, but WILL warn
    } else {
        callback();    // this is fine, but WILL warn
    }
}

When Not To Use It

There are some cases where you might want to call a callback function more than once. In those cases this rule may lead to incorrect behavior. In those cases you may want to reserve a special name for those callbacks and not include that in the list of callbacks that trigger warnings.

Further Reading

• The Art Of Node: Callbacks
• Nodejitsu: What are the error conventions?

Related Rules

• [handle-callback-err](handle-callback-err.md) Source: http://eslint.org/docs/rules/

      require(['marked'], function (marked) { //load lib on-demand to improve performance

Close

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/

          a = s.createElement(o), m = s.getElementsByTagName(o)[0];

Close

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 to true will allow you to use short circuit evaluations in your expressions (Default: false).
• allowTernary set to true will enable you to use ternary operators in your expressions similarly to short circuit evaluations (Default: false).
• allowTaggedTemplates set to true 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/