function foo(message, delay) {
    return new Promise(resolve => {
        setTimeout(() => {
            console.log(`${message} after sleeping for ${delay}ms`)
            resolve()
        }, delay)
    })
}

// Instead of doing this:

// async function callAsync(callable, n) {
//     for (let i = 0; i < n; i++) {
//         await callable(`X${i}`, 5)
//     }
// }

// on a non async/await supported browser, you can do this:
function callAsync(callable, n) {
    return new Promise(resolve => {
        let i = 0
        let prom
        const thenHandler = () => {
            if (i === n) {
                resolve()
            } else {
                i++
                prom = foo(`X${i}`, 5).then(thenHandler())
            }
        }
        prom = foo(`X${i}`, 5).then(() => thenHandler())
    })
}

callAsync(foo, 15).then(() => console.log("done"))