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Add a window property with TypeScript

In TypeScript, you can define custom properties on the `window` namespace by declaring them as global properties. For example, if you want to make TypeScript aware of the property `window.__coverage__`, you can declare it in your code using the `declare global` syntax.

Sometimes you want to define a custom property on the window namespace. This can be done by declaring it as a global property.

Contents

Define a window property

In the following case we want to make TypeScript aware of the property window.__coverage__ which is set by istanbul's instrumenter class.

When calling __coverage__ on window the TypeScript compiler complains with the following error:

TS2339: Property 'coverage' does not exist on type 'Window'.

To get along with it we have to make TypeScript aware of this new property by declaring it in our code:

declare global {
  interface Window {
    __coverage__: Object;
  }
}

Define a global property

If you want to define a global __coverage__ property in a Node.js environment, you will have to assign it to the global object because there is no window in Node.js:

declare global {
  namespace NodeJS {
    interface Global {
      __coverage__: {};
    }
  }
}
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