_JavaScript_enumerate_iterable

enumerate.js

function *enumerate(init=0, iter) {
    if (init[Symbol.iterator]) {
        iter = init[Symbol.iterator]();
        init = 0;
    }

    let count = init;
    for(const a of iter) {
        yield [count, a];
        count += 1;
    }
}

console.log(...enumerate([10, 20, 30])); // [ 0, 10 ] [ 1, 20 ] [ 2, 30 ]
console.log(...enumerate(5, [10, 20, 30])); // [ 5, 10 ] [ 6, 20 ] [ 7, 30 ]