js/rest-spread-spacing

ES2015 introduced the rest and spread operators, which expand an iterable structure into its individual parts. Some examples of their usage are as follows:

let numArr = [1, 2, 3];
function add(a, b, c) {
    return a + b + c;
}
add(...numArr); // -> 6

let arr1 = [1, 2, 3];
let arr2 = [4, 5, 6];
arr1.push(...arr2); // -> [1, 2, 3, 4, 5, 6]

let [a, b, ...arr] = [1, 2, 3, 4, 5];
a; // -> 1
b // -> 2
arr; // ->  [3, 4, 5]

function numArgs(...args) {
  return args.length;
}
numArgs(a, b, c); // -> 3

In addition to the above, there is currently a proposal to add object rest and spread properties to the spec. They can be used as follows:

let { x, y, ...z } = { x: 1, y: 2, a: 3, b: 4 };
x; // -> 1
y; // -> 2
z; // -> { a: 3, b: 4 }

let n = { x, y, ...z };
n; // -> { x: 1, y: 2, a: 3, b: 4 }

As with other operators, whitespace is allowed between the rest or spread operator and the expression it is operating on, which can lead to inconsistent spacing within a codebase.

Rule Details

This rule aims to enforce consistent spacing between rest and spread operators and their expressions. The rule also supports object rest and spread properties in ES2018:

{
    "parserOptions": {
        "ecmaVersion": 2018
    }
}

Please read the user guide's section on configuring parser options to learn more.

Options

This rule takes one option: a string with the value of "never" or "always". The default value is "never".

"never"

When using the default "never" option, whitespace is not allowed between spread operators and their expressions.

rest-spread-spacing: ["error"]

or

rest-spread-spacing: ["error", "never"]

Examples of incorrect code for this rule with "never":

::: incorrect

/*eslint rest-spread-spacing: ["error", "never"]*/

fn(... args);
[... arr, 4, 5, 6];
let [a, b, ... arr] = [1, 2, 3, 4, 5];
function fn(... args) { console.log(args); }
let { x, y, ... z } = { x: 1, y: 2, a: 3, b: 4 };
let n = { x, y, ... z };

:::

Examples of correct code for this rule with "never":

::: correct

/*eslint rest-spread-spacing: ["error", "never"]*/

fn(...args);
[...arr, 4, 5, 6];
let [a, b, ...arr] = [1, 2, 3, 4, 5];
function fn(...args) { console.log(args); }
let { x, y, ...z } = { x: 1, y: 2, a: 3, b: 4 };
let n = { x, y, ...z };

:::

"always"

When using the "always" option, whitespace is required between spread operators and their expressions.

rest-spread-spacing: ["error", "always"]

Examples of incorrect code for this rule with "always":

::: incorrect

/*eslint rest-spread-spacing:["error", "always"]*/

fn(...args);
[...arr, 4, 5, 6];
let [a, b, ...arr] = [1, 2, 3, 4, 5];
function fn(...args) { console.log(args); }
let { x, y, ...z } = { x: 1, y: 2, a: 3, b: 4 };
let n = { x, y, ...z };

:::

Examples of correct code for this rule with "always":

::: correct

/*eslint rest-spread-spacing: ["error", "always"]*/

fn(... args);
[... arr, 4, 5, 6];
let [a, b, ... arr] = [1, 2, 3, 4, 5];
function fn(... args) { console.log(args); }
let { x, y, ... z } = { x: 1, y: 2, a: 3, b: 4 };
let n = { x, y, ... z };

:::

When Not To Use It

You can safely disable this rule if you do not care about enforcing consistent spacing between spread operators and their expressions.