SOURCE

function All(arr) {
    let len = arr.length;
    let results = []
    for (let i = 0; i < len; i++) {
        if (i === len) {
            return resolve(results)
        }
    }
}

function a(num) {
    return new Promise((resolve) => {
        setTimeout(() => {
            resolve(num);
        }, 1000)
    })
}

All([a(100), a(200)]);