no-top-level-variables.md

No top level variables (no-top-level-variables)

Disallow top level variables.

Variables at the top level may indicate side effects because it may be used as state in functions or methods. As such, const is the only kind of top-level variable allowed by default, and it can only be assigned certain values.

Rule Details

This rule lets you control top level variables.

Examples of incorrect code for this rule:

var answer = 42;
let foo = 'bar';
const arr = [];

Examples of correct code for this rule:

import * as util from 'node:util';

const leet = 1337;
const l33t = leet;

function f() {
  var answer = 42;
}

const g = () => {
  let foo = 'bar';
};

const h = function () {
  const arr = [];
};

export default function () {
  // ...
}

export const foo = 'bar';

const util = require('node:util');

module.exports = {
  foo: 'bar'
};

Options

This rule accepts a configuration object with two options:

• allowed: Configure what kind of assignments are allowed. Some assignments are always allowed, others need to be allowed explicitly.
• kind: Configure which kinds of variables are allowed. By default only const variables are allowed.

allowed

Examples of correct code when 'ArrayExpression' is in the list:

const arr = [1, 2, 3];

Examples of correct code when 'ObjectExpression' is in the list:

const hello = {world: '!'};

Additionally, all others expression types that aren't always allowed can be allowed, those are ImportExpression, SequenceExpression, ThisExpression, YieldExpression.

kind

Examples of correct code when 'const' is in the list:

const answer = 42;
const foo = 'bar';
const path = require('path');

Examples of correct code when 'let' is in the list:

let answer = 42;
let foo = 'bar';
let path = require('path');

Examples of correct code when 'var' is in the list:

var answer = 42;
var foo = 'bar';
var path = require('path');

Unless there is a historical or compatibility reason to allow var or let, it is recommended to only allow const. By setting this to an empty list you can disallow all top-level variables.

When Not To Use It

If you want to allow top level variables.

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