globalAssignments

Reports attempting to assign to read-only global variables such as undefined, NaN, Infinity, Object, etc.

✅ This rule is included in the ts untyped presets.

Global variables like undefined, NaN, Infinity, and built-in objects like Object and Array are read-only and should not be modified. Attempting to reassign these globals can lead to confusing behavior and potential bugs in your code. In strict mode, reassigning these globals will throw a TypeError at runtime.

undefined = 1;Error ts(2539)  ― Cannot assign to 'undefined' because it is not a variable.

var NaN: number
NaN = 42;

var Infinity: number
Infinity = 100;

Object = null;Error ts(2322)  ― Type 'null' is not assignable to type 'ObjectConstructor'.

Array = function () {};Error ts(2739)  ― Type '() => void' is missing the following properties from type 'ArrayConstructor': isArray, from, of, [Symbol.species]

let 
let count: number
count = 5;
undefined += 
let count: number
count;Error ts(2539)  ― Cannot assign to 'undefined' because it is not a variable.

var NaN: number
NaN++;

--
var Infinity: number
Infinity;

let 
let myUndefined: number
myUndefined = 1;

const 
const notANumber: 42
notANumber = 42;

const 
const infiniteValue: 100
infiniteValue = 100;

let 
let myObject: null
myObject = null;

const 
const myArray: () => void
myArray = function () {};

let 
let count: number
count = 5;
let 
let undefinedValue: undefined
undefinedValue = 
var undefined
undefined;
undefinedValue = 
let count: number
count;Error ts(2322)  ― Type 'number' is not assignable to type 'undefined'.

let 
let notANumber: number
notANumber = 
var NaN: number
NaN;
let notANumber: number
notANumber++;

let 
let infiniteValue: number
infiniteValue = 
var Infinity: number
Infinity;
--
let infiniteValue: number
infiniteValue;

Options

This rule is not configurable.

When Not To Use It

If your codebase intentionally assigns to global objects and is guaranteed to run in an environment that allows that, you might not want to enable this rule. For example, if you target a legacy runtime with legacy non-standard JavaScript semantics, modern standard practices may not apply to you.

Equivalents in Other Linters

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globalAssignments

Examples

Options

When Not To Use It

Further Reading

Equivalents in Other Linters