Extending Global or Third Party Library Typing

When you install TypeScript in your project, it comes with many declarations that is part of JavaScript standard, e.g. Array.prototype.map, and Object.keys, so Intellisense just works when you use those standard JavaScript object method.

However, assuming that your company extends those global object with its own custom methods (I am not saying this is a good idea), how do you make TypeScript recognize it?

Since extending object in JavaScript is something many people used to do, TypeScript provides you a mechanism to do it. Just like you can merge JavaScript object, you can merge declaration in TypeScript.

Extend Global Object & Declarations

To extends global object e.g. Array, include the following code in any TypeScript file in your project:

declare global {
interface Array<T> {
fly: () => void;

Intellisense showing additional array methods

Because TypeScript also declare Array under global, when it sees the code above, it will interpret that as your intention to add additional methods to Array, so it merge that into the original Array.prototype declaration.

The same approach is used to add your custom web component tag so TypeScript allow it to be part of JSX:

declare global {
namespace JSX {
interface IntrinsicElements {
'my-custom-component': any;

Then you can render that it in your React component:

const MyComponent: React.FC = () => (

Extend Third Party Declaration

Now assuming you're using some third-party libraries that allows you to extend it, e.g. Jest allows you to extends its expect assertion, how do you make its declaration includes your customization as well?

Assuming I want to add a toBeAwesome assertion, following is how I do it:

declare global {
namespace jest {
interface Matchers<R> {
toBeAwesome(): R;

Intellisense showing toBeAwesome

It is awesome, isn't it?


In short, to extend global or third-party declaration, usually you need to additional declaration to make TypeScript merge that in.

To do that, you need to find out how the current declaration is being declared. If it is declared as a global interface, declare a global interface (like Array); if it is declared under a namespace, declare that under the namespace too (like JSX.IntrinsicElements and jest.Matcher).


  • typescript