Here’s the fun part that I suspect many here are forgetting: if you want to write the function body, it will probably (or at the very least can) look very simil

Here’s the fun part that I suspect many here are forgetting: if you want to write the function body, it will probably (or at the very least can) look very similar! function flat() { // type FlatArr<Arg extends unknown[]> = let [first, ...rest] = this; return (this.length > 0) ? // Arg extends [infer First, ...(infer Rest)] ? Array.isArray(first) ? // First extends unknown[] ? [...first, ...flat(rest)] : // [...First, ...FlatArr<Rest>] : [first, ...flat(rest)] : // [First, ...FlatArr<Rest>] : []; // []; } If you want to unternarise it: function flat() { if (this.length > 0) { let [first, ...rest] = this; if (Array.isArray(first)) { return [...first, ...flat(rest)]; } else { return [first, ...flat(rest)]; } } else { return []; } } I still wouldn’t call it basic TypeScript, but it’s not conceptually that advanced, you just need to know about infer and extends. Now in reality, Array.prototype.flat has a more complex definition, partly because (like most of Array’s methods) the method is generic (it works on array-like objects that have a length property and numeric indexing), and partly because of the depth parameter. From lib.es2019.array.d.ts: type FlatArray<Arr, Depth extends number> = { done: Arr; recur: Arr extends ReadonlyArray<infer InnerArr> ? FlatArray<InnerArr, [-1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20][Depth]> : Arr; }[Depth extends -1 ? "done" : "recur"]; interface ReadonlyArray<T> { // [Snipped: flatMap, and flat’s doc comment] flat<A, D extends number = 1>( this: A, depth?: D, ): FlatArray<A, D>[]; } // [And repeated for interface Array<T>.] Ouch. Don’t like the { done, recur }[Depth extends -1 ? "done" : "recur"] at all, no idea why it wasn’t written as `Depth extends -1 ? Arr : Arr extends ReadonlyArray<…`. And as for hard-coding support for depths up to 20 then bailing… probably pragmatic, it’s possible to support all values, but rather messy: https://stackoverflow.com/q/54243431 https://stackoverflow.com/q/54243431.