Require CamelCase (camelcase)

When it comes to naming variables, style guides generally fall into one of two camps: camelcase (variableName) and underscores (variable_name). This rule focuses on using the camelcase approach. If your style guide calls for camelCasing your variable names, then this rule is for you!

Rule Details

This rule looks for any underscores (_) located within the source code. It ignores leading and trailing underscores and only checks those in the middle of a variable name. If ESLint decides that the variable is a constant (all uppercase), then no warning will be thrown. Otherwise, a warning will be thrown. This rule only flags definitions and assignments but not function calls. In case of ES6 import statements, this rule only targets the name of the variable that will be imported into the local module scope.

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names
• "ignoreDestructuring": false (default) enforces camelcase style for destructured identifiers
• "ignoreDestructuring": true does not check destructured identifiers
• allow (string[]) list of properties to accept. Accept regex.

properties: "always"

Examples of incorrect code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

obj.do_something = function() {
    // ...
};

function foo({ no_camelcased }) {
    // ...
};

function foo({ isCamelcased: no_camelcased }) {
    // ...
}

function foo({ no_camelcased = 'default value' }) {
    // ...
};

var obj = {
    my_pref: 1
};

var { category_id = 1 } = query;

var { foo: no_camelcased } = bar;

var { foo: bar_baz = 1 } = quz;

Examples of correct code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased as camelCased } from "external-module";

var myFavoriteColor   = "#112C85";
var _myFavoriteColor  = "#112C85";
var myFavoriteColor_  = "#112C85";
var MY_FAVORITE_COLOR = "#112C85";
var foo = bar.baz_boom;
var foo = { qux: bar.baz_boom };

obj.do_something();
do_something();
new do_something();

var { category_id: category } = query;

function foo({ isCamelCased }) {
    // ...
};

function foo({ isCamelCased: isAlsoCamelCased }) {
    // ...
}

function foo({ isCamelCased = 'default value' }) {
    // ...
};

var { categoryId = 1 } = query;

var { foo: isCamelCased } = bar;

var { foo: isCamelCased = 1 } = quz;

properties: "never"

Examples of correct code for this rule with the { "properties": "never" } option:

/*eslint camelcase: ["error", {properties: "never"}]*/

var obj = {
    my_pref: 1
};

ignoreDestructuring: false

Examples of incorrect code for this rule with the default { "ignoreDestructuring": false } option:

/*eslint camelcase: "error"*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

var { category_id: category_alias } = query;

var { category_id: categoryId, ...other_props } = query;

ignoreDestructuring: true

Examples of incorrect code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id: category_alias } = query;

var { category_id, ...other_props } = query;

Examples of correct code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

allow

Examples of correct code for this rule with the allow option:

/*eslint camelcase: ["error", {allow: ["UNSAFE_componentWillMount"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

/*eslint camelcase: ["error", {allow: ["^UNSAFE_"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

function UNSAFE_componentWillMount() {
    // ...
}

When Not To Use It

If you have established coding standards using a different naming convention (separating words with underscores), turn this rule off. Source: http://eslint.org/docs/rules/

Require CamelCase (camelcase)

When it comes to naming variables, style guides generally fall into one of two camps: camelcase (variableName) and underscores (variable_name). This rule focuses on using the camelcase approach. If your style guide calls for camelCasing your variable names, then this rule is for you!

Rule Details

This rule looks for any underscores (_) located within the source code. It ignores leading and trailing underscores and only checks those in the middle of a variable name. If ESLint decides that the variable is a constant (all uppercase), then no warning will be thrown. Otherwise, a warning will be thrown. This rule only flags definitions and assignments but not function calls. In case of ES6 import statements, this rule only targets the name of the variable that will be imported into the local module scope.

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names
• "ignoreDestructuring": false (default) enforces camelcase style for destructured identifiers
• "ignoreDestructuring": true does not check destructured identifiers
• allow (string[]) list of properties to accept. Accept regex.

properties: "always"

Examples of incorrect code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

obj.do_something = function() {
    // ...
};

function foo({ no_camelcased }) {
    // ...
};

function foo({ isCamelcased: no_camelcased }) {
    // ...
}

function foo({ no_camelcased = 'default value' }) {
    // ...
};

var obj = {
    my_pref: 1
};

var { category_id = 1 } = query;

var { foo: no_camelcased } = bar;

var { foo: bar_baz = 1 } = quz;

Examples of correct code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased as camelCased } from "external-module";

var myFavoriteColor   = "#112C85";
var _myFavoriteColor  = "#112C85";
var myFavoriteColor_  = "#112C85";
var MY_FAVORITE_COLOR = "#112C85";
var foo = bar.baz_boom;
var foo = { qux: bar.baz_boom };

obj.do_something();
do_something();
new do_something();

var { category_id: category } = query;

function foo({ isCamelCased }) {
    // ...
};

function foo({ isCamelCased: isAlsoCamelCased }) {
    // ...
}

function foo({ isCamelCased = 'default value' }) {
    // ...
};

var { categoryId = 1 } = query;

var { foo: isCamelCased } = bar;

var { foo: isCamelCased = 1 } = quz;

properties: "never"

Examples of correct code for this rule with the { "properties": "never" } option:

/*eslint camelcase: ["error", {properties: "never"}]*/

var obj = {
    my_pref: 1
};

ignoreDestructuring: false

Examples of incorrect code for this rule with the default { "ignoreDestructuring": false } option:

/*eslint camelcase: "error"*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

var { category_id: category_alias } = query;

var { category_id: categoryId, ...other_props } = query;

ignoreDestructuring: true

Examples of incorrect code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id: category_alias } = query;

var { category_id, ...other_props } = query;

Examples of correct code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

allow

Examples of correct code for this rule with the allow option:

/*eslint camelcase: ["error", {allow: ["UNSAFE_componentWillMount"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

/*eslint camelcase: ["error", {allow: ["^UNSAFE_"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

function UNSAFE_componentWillMount() {
    // ...
}

When Not To Use It

If you have established coding standards using a different naming convention (separating words with underscores), turn this rule off. Source: http://eslint.org/docs/rules/

Require CamelCase (camelcase)

When it comes to naming variables, style guides generally fall into one of two camps: camelcase (variableName) and underscores (variable_name). This rule focuses on using the camelcase approach. If your style guide calls for camelCasing your variable names, then this rule is for you!

Rule Details

This rule looks for any underscores (_) located within the source code. It ignores leading and trailing underscores and only checks those in the middle of a variable name. If ESLint decides that the variable is a constant (all uppercase), then no warning will be thrown. Otherwise, a warning will be thrown. This rule only flags definitions and assignments but not function calls. In case of ES6 import statements, this rule only targets the name of the variable that will be imported into the local module scope.

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names
• "ignoreDestructuring": false (default) enforces camelcase style for destructured identifiers
• "ignoreDestructuring": true does not check destructured identifiers
• allow (string[]) list of properties to accept. Accept regex.

properties: "always"

Examples of incorrect code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

obj.do_something = function() {
    // ...
};

function foo({ no_camelcased }) {
    // ...
};

function foo({ isCamelcased: no_camelcased }) {
    // ...
}

function foo({ no_camelcased = 'default value' }) {
    // ...
};

var obj = {
    my_pref: 1
};

var { category_id = 1 } = query;

var { foo: no_camelcased } = bar;

var { foo: bar_baz = 1 } = quz;

Examples of correct code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased as camelCased } from "external-module";

var myFavoriteColor   = "#112C85";
var _myFavoriteColor  = "#112C85";
var myFavoriteColor_  = "#112C85";
var MY_FAVORITE_COLOR = "#112C85";
var foo = bar.baz_boom;
var foo = { qux: bar.baz_boom };

obj.do_something();
do_something();
new do_something();

var { category_id: category } = query;

function foo({ isCamelCased }) {
    // ...
};

function foo({ isCamelCased: isAlsoCamelCased }) {
    // ...
}

function foo({ isCamelCased = 'default value' }) {
    // ...
};

var { categoryId = 1 } = query;

var { foo: isCamelCased } = bar;

var { foo: isCamelCased = 1 } = quz;

properties: "never"

Examples of correct code for this rule with the { "properties": "never" } option:

/*eslint camelcase: ["error", {properties: "never"}]*/

var obj = {
    my_pref: 1
};

ignoreDestructuring: false

Examples of incorrect code for this rule with the default { "ignoreDestructuring": false } option:

/*eslint camelcase: "error"*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

var { category_id: category_alias } = query;

var { category_id: categoryId, ...other_props } = query;

ignoreDestructuring: true

Examples of incorrect code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id: category_alias } = query;

var { category_id, ...other_props } = query;

Examples of correct code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

allow

Examples of correct code for this rule with the allow option:

/*eslint camelcase: ["error", {allow: ["UNSAFE_componentWillMount"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

/*eslint camelcase: ["error", {allow: ["^UNSAFE_"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

function UNSAFE_componentWillMount() {
    // ...
}

When Not To Use It

If you have established coding standards using a different naming convention (separating words with underscores), turn this rule off. Source: http://eslint.org/docs/rules/

Suggest using template literals instead of string concatenation. (prefer-template)

In ES2015 (ES6), we can use template literals instead of string concatenation.

var str = "Hello, " + name + "!";

/*eslint-env es6*/

var str = `Hello, ${name}!`;

Rule Details

This rule is aimed to flag usage of + operators with strings.

Examples

Examples of incorrect code for this rule:

/*eslint prefer-template: "error"*/

var str = "Hello, " + name + "!";
var str = "Time: " + (12 * 60 * 60 * 1000);

Examples of correct code for this rule:

/*eslint prefer-template: "error"*/
/*eslint-env es6*/

var str = "Hello World!";
var str = `Hello, ${name}!`;
var str = `Time: ${12 * 60 * 60 * 1000}`;

// This is reported by `no-useless-concat`.
var str = "Hello, " + "World!";

When Not To Use It

This rule should not be used in ES3/5 environments.

In ES2015 (ES6) or later, if you don't want to be notified about string concatenation, you can safely disable this rule.

Related Rules

• [no-useless-concat](no-useless-concat.md)
• [quotes](quotes.md) Source: http://eslint.org/docs/rules/

require let or const instead of var (no-var)

ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let and const keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes such as:

var count = people.length;
var enoughFood = count > sandwiches.length;

if (enoughFood) {
    var count = sandwiches.length; // accidentally overriding the count variable
    console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}

// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");

Rule Details

This rule is aimed at discouraging the use of var and encouraging the use of const or let instead.

Examples

Examples of incorrect code for this rule:

/*eslint no-var: "error"*/

var x = "y";
var CONFIG = {};

Examples of correct code for this rule:

/*eslint no-var: "error"*/
/*eslint-env es6*/

let x = "y";
const CONFIG = {};

When Not To Use It

In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their codebase may not want to apply this rule if the cost of migrating from var to let is too costly. Source: http://eslint.org/docs/rules/

require return statements to either always or never specify values (consistent-return)

Unlike statically-typed languages which enforce that a function returns a specified type of value, JavaScript allows different code paths in a function to return different types of values.

A confusing aspect of JavaScript is that a function returns undefined if any of the following are true:

• it does not execute a return statement before it exits
• it executes return which does not specify a value explicitly
• it executes return undefined
• it executes return void followed by an expression (for example, a function call)
• it executes return followed by any other expression which evaluates to undefined

If any code paths in a function return a value explicitly but some code path do not return a value explicitly, it might be a typing mistake, especially in a large function. In the following example:

• a code path through the function returns a Boolean value true
• another code path does not return a value explicitly, therefore returns undefined implicitly

function doSomething(condition) {
    if (condition) {
        return true;
    } else {
        return;
    }
}

Rule Details

This rule requires return statements to either always or never specify values. This rule ignores function definitions where the name begins with an uppercase letter, because constructors (when invoked with the new operator) return the instantiated object implicitly if they do not return another object explicitly.

Examples of incorrect code for this rule:

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

function doSomething(condition) {
    if (condition) {
        return true;
    } else {
        return;
    }
}

function doSomething(condition) {
    if (condition) {
        return true;
    }
}

Examples of correct code for this rule:

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

function doSomething(condition) {
    if (condition) {
        return true;
    } else {
        return false;
    }
}

function Foo() {
    if (!(this instanceof Foo)) {
        return new Foo();
    }

    this.a = 0;
}

Options

This rule has an object option:

• "treatUndefinedAsUnspecified": false (default) always either specify values or return undefined implicitly only.
• "treatUndefinedAsUnspecified": true always either specify values or return undefined explicitly or implicitly.

treatUndefinedAsUnspecified

Examples of incorrect code for this rule with the default { "treatUndefinedAsUnspecified": false } option:

/*eslint consistent-return: ["error", { "treatUndefinedAsUnspecified": false }]*/

function foo(callback) {
    if (callback) {
        return void callback();
    }
    // no return statement
}

function bar(condition) {
    if (condition) {
        return undefined;
    }
    // no return statement
}

Examples of incorrect code for this rule with the { "treatUndefinedAsUnspecified": true } option:

/*eslint consistent-return: ["error", { "treatUndefinedAsUnspecified": true }]*/

function foo(callback) {
    if (callback) {
        return void callback();
    }
    return true;
}

function bar(condition) {
    if (condition) {
        return undefined;
    }
    return true;
}

Examples of correct code for this rule with the { "treatUndefinedAsUnspecified": true } option:

/*eslint consistent-return: ["error", { "treatUndefinedAsUnspecified": true }]*/

function foo(callback) {
    if (callback) {
        return void callback();
    }
    // no return statement
}

function bar(condition) {
    if (condition) {
        return undefined;
    }
    // no return statement
}

When Not To Use It

If you want to allow functions to have different return behavior depending on code branching, then it is safe to disable this rule. Source: http://eslint.org/docs/rules/