How to declare the type of 'this' in a TypeScript function?

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Sometimes, we want to declare the type of ‘this’ in a TypeScript function.

In this article, we’ll look at how to declare the type of ‘this’ in a TypeScript function.

To declare the type of ‘this’ in a TypeScript function, we can add the fake this parameter in our functions.

For instance, we write

class Handler {
  info: string;

  onClick(this: Handler, e: Event) {
    this.info = e.message;
  }
}

const h = new Handler();

to add the this parameter into onClick and set it to type Handler.

The parameter will be discarded after it’s compiled.

But it’ll let the TypeScript compiler check for valid properties for this.

To declare the type of ‘this’ in a TypeScript function, we can add the fake this parameter in our functions.

By John Au-Yeung