How to Properly Type Window

In pretty much any frontend application, you'll likely have encountered this error:

Property 'X' does not exist on type 'Window & typeof globalThis'.

window.X;
Property 'X' does not exist on type 'Window & typeof globalThis'.2339
Property 'X' does not exist on type 'Window & typeof globalThis'.

In this article, we'll lay out several different solutions to this issue.

#

Quick Fix

The Window interface is defined globally in a file called lib.dom.d.ts. You can change it using various techniques:

• To change it globally within a .ts or .tsx file, you can use declare global and interface Window:

declare global {
  interface Window {
    X: number;
  }
}

window.X;
      
(property) Window.X: number

• To change it globally inside a .d.ts file, you can just specify interface Window.

interface Window {
  X: number;
}

• To change it for one file only, you can use declare const window in a .ts or .tsx file:

declare const window: {
  X: number;
} & Window;

window.X;
      
(property) X: number

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Explanation

The interface Window lives in the global scope in TypeScript. It ships as part of the DOM types in lib.dom.d.ts, which describes what methods are available in the browser.

The issue comes up when a third-party script (or perhaps our own code) adds something to the window:

window.X = Date.now();

This is a problem because TypeScript doesn't know about the X property, and so it throws an error when you try to access it:

window.X;
Property 'X' does not exist on type 'Window & typeof globalThis'.2339
Property 'X' does not exist on type 'Window & typeof globalThis'.

So, we need to somehow change the global definition of Window to include the new property that TypeScript doesn't know about.

#

Solution 1: declare global in a .ts or .tsx file

The first solution is to use declare global to change the global definition of Window:

declare global {
  interface Window {
    X: number;
  }
}

window.X;
      
(property) Window.X: number

This works for two reasons. First, declare global tells TypeScript that anything inside it should be added to the global scope.

Second, we take advantage of declaration merging. This is a rule in TypeScript where interfaces with the same name in the same scope get merged. So by redeclaring Window in the same scope, we can append new properties to it.

This wouldn't work if we used type because types don't support declaration merging.

declare global {
  type Window = {
Duplicate identifier 'Window'.2300
Duplicate identifier 'Window'.    X: number;
  };
}

This solution only works inside a .ts or .tsx file because declare global only works inside a module. So this solution is a little awkward in terms of where you place the definition in your project. In its own module? Colocated with something else?

#

Solution 2: A .d.ts file

A neater solution is to use interface Window in a .d.ts file:

// window.d.ts (choose any filename you like)

interface Window {
  X: number;
}

// your-app.ts
window.X;
      
(property) Window.X: number

This works because anything you place in a .d.ts automatically goes into the global scope, so there's no need for declare global.

It also lets you place the global definition in a single file, on its own, which feels a little cleaner than trying to figure out which .ts file to put it in.

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Solution 3: Single-module override

If you're concerned about adding types into the global scope, you can use declare const window to override the type of window in a single module:

declare const window: {
  X: number;
} & Window;

window.X;
      
(property) X: number

declare const window acts like const window, but on the type level. So it only acts on the current scope — in this case, the module. We declare the type as the current Window, plus the new property X.

This solution gets a little annoying if you need to access window.X in multiple files because you'll need to add the declare const window line to each file.

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Which solution should I use?

Personally, I tend to reach for solution 2 — a .d.ts file. It's the fewest lines of code and the easiest to place in your project.

I don't mind adding types into the global scope. By actually changing the type of Window, you're more accurately describing the environment your code executes in. In my book, that's a bonus.

But if you're really concerned about it, use the declare const solution.