function intersperse<T>(this: T[], mkT: (ix: number) => T): T[] { return this.reduce((acc: T[], d, ix) => [...acc, mkT(ix), d], []).slice(1); }

Array.from(Array(10).keys()) //=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

An array is from a logical point of view not an object - although JavaScript handles and reports them as such. In practice however, it is not helpful to see them equal, because they are not.

Arrays are definitely objects. Not sure why you think objects can't have a length property nor methods like push, Object.create(Array.prototype) is a trivial counterexample of a non-array object which has these. What makes arrays special is that they are exotic objects with a custom [[DefineOwnProperty]] essential internal method, but they are still objects.

function* enumerate(iterable) { let i = 0; for (const x of iterable) { yield [i, x]; i++; } } for (const [i, obj] of enumerate(myArray)) { console.log(i, obj); }

for (let [index, val] of array.entries())

Note that Array.entries() returns an iterator, which is what allows it to work in the for-of loop; don't confuse this with Object.entries(), which returns an array of key-value pairs.