init-declarations

Require or disallow initialization in variable declarations

In JavaScript, variables can be assigned during declaration, or at any point afterwards using an assignment statement. For example, in the following code, foo is initialized during declaration, while bar is initialized later.

let foo = 1;
let bar;

if (foo) {
    bar = 1;
} else {
    bar = 2;
}
1
2
3
4
5
6
7
8

Rule Details

This rule is aimed at enforcing or eliminating variable initializations during declaration. For example, in the following code, foo is initialized during declaration, while bar is not.

let foo = 1;
let bar;

bar = 2;
1
2
3
4

This rule aims to bring consistency to variable initializations and declarations.

Options

The rule takes two options:

1. A string which must be either "always" (the default), to enforce initialization at declaration, or "never" to disallow initialization during declaration. This rule applies to var, let, and const variables, however "never" is ignored for const variables, as unassigned consts generate a parse error.
2. An object that further controls the behavior of this rule. Currently, the only available parameter is ignoreForLoopInit, which indicates if initialization at declaration is allowed in for loops when "never" is set, since it is a very typical use case.

You can configure the rule as follows:

Variables must be initialized at declaration (default)

{
    "init-declarations": ["error", "always"],
}
1
2
3

Variables must not be initialized at declaration

{
    "init-declarations": ["error", "never"]
}
1
2
3

Variables must not be initialized at declaration, except in for loops, where it is allowed

{
    "init-declarations": ["error", "never", { "ignoreForLoopInit": true }]
}
1
2
3

always

Examples of incorrect code for the default "always" option:

/*eslint init-declarations: ["error", "always"]*/

function foo() {
    var bar;
    let baz;
}
1
2
3
4
5
6

Examples of correct code for the default "always" option:

/*eslint init-declarations: ["error", "always"]*/

function foo() {
    var bar = 1;
    let baz = 2;
    const qux = 3;
}
1
2
3
4
5
6
7

never

Examples of incorrect code for the "never" option:

/*eslint init-declarations: ["error", "never"]*/

function foo() {
    var bar = 1;
    let baz = 2;

    for (let i = 0; i < 1; i++) {}
}
1
2
3
4
5
6
7
8

Examples of correct code for the "never" option:

/*eslint init-declarations: ["error", "never"]*/

function foo() {
    var bar;
    let baz;
    const buzz = 1;
}
1
2
3
4
5
6
7

The "never" option ignores const variable initializations.

ignoreForLoopInit

Examples of correct code for the "never", { "ignoreForLoopInit": true } options:

/*eslint init-declarations: ["error", "never", { "ignoreForLoopInit": true }]*/
for (let i = 0; i < 1; i++) {}
1
2

This rule additionally supports TypeScript type syntax.

Examples of incorrect TypeScript code for this rule:

/* eslint init-declarations: ["error", "never"] */

let arr: string[] = ['arr', 'ar'];

const class1 = class NAME {
	constructor() {
	  var name1: string = 'hello';
	}
};

namespace myLib {
	let numberOfGreetings: number = 2;
}

1
2
3
4
5
6
7
8
9
10
11
12
13
14

/* eslint init-declarations: ["error", "always"] */

namespace myLib {
	let numberOfGreetings: number;
}

namespace myLib1 {
	const foo: number;
	namespace myLib2 {
	  let bar: string;
	  namespace myLib3 {
		let baz: object;
	  }
	}
}

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16

Examples of correct TypeScript code for this rule:

/* eslint init-declarations: ["error", "never"] */

declare const foo: number;

declare namespace myLib {
	let numberOfGreetings: number;
}

interface GreetingSettings {
	greeting: string;
	duration?: number;
	color?: string;
}
1
2
3
4
5
6
7
8
9
10
11
12
13

/* eslint init-declarations: ["error", "always"] */

declare const foo: number;

declare namespace myLib {
	let numberOfGreetings: number;
}

interface GreetingSettings {
	greeting: string;
	duration?: number;
	color?: string;
}

1
2
3
4
5
6
7
8
9
10
11
12
13
14

When Not To Use It

When you are indifferent as to how your variables are initialized.

Version

This rule was introduced in ESLint v1.0.0-rc-1.

Resources

• Rule source
• Tests source