Array.prototype.flatMap method in TypeScript.

flatMap.ts

interface Array<T> {
    flatMap<E>(callback: (t: T) => Array<E>): Array<E>
}

Object.defineProperty(Array.prototype, 'flatMap', {
    value: function(f: Function) {
        return this.reduce((ys: any, x: any) => {
            return ys.concat(f.call(this, x))
        }, [])
    },
    enumerable: false,
})