Extending the Array.prototype

Question

I can extend Array.prototype and use it in javascript like that:

Object.defineProperty(Array.prototype, 'flatten', {
  value: function flatten() {
      return this.reduce((a, b) => a.concat(b), []);
  }
});

// that allows me to do:
[1, 2, [3, 4]].flatten(); // [1,2,3,4]
[1, 2, [3, 4]].map(e => 5).flatten(); // [5, 5, 5]

However under typescript I get an Error: Property 'flatten' does not exist on type 'Array[]'.

I can avoid that doing something like the following, but that is not nice to get a fluent interface...it's harder to read.

 (<any>[1, 2, [3, 4]]).flatten()
 ([1, 2, [3, 4]].map(e => 5) as any).flatten()

Can I do something in typescript to avoid a casting to any? Like let to know typescript dynamically than the code of type Array has another method.

Saravana · Accepted Answer · 2017-07-05 10:17:35Z

4

You can extend the Array<T> interface to add the flatten method:

declare global {
    interface Array<T> {
        flatten(): T[]
    }
}

Note that you only need to wrap it inside declare global only when you are inside a module. See merging interfaces.

answered Jul 5, 2017 at 10:17

Saravana

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