Async Javascript experiments

async-experiments.js

function sleep(ms) {
    return new Promise(resolve => setTimeout(resolve, ms));
}

/**
 * Shows how a timer fails to execute when we block the main thread. See `setIntervalDoesNotWorkWithBlockingWait()` for
 * a solution to this.
 * @returns {void}
 */
function setIntervalDoesNotWorkWithBlockingWait() {
    setInterval(() => console.info("hello"), 1000);
    const end = Date.now() + 3000;
    console.info("Going to sleep");
    while (Date.now() < end) {
        // hello won't be printed while this loop is busy-waiting!
    }
    console.info("Back from sleep");
}

/**
 * Shows how a timer can still work even though we are sleeping, given that we `await` rather than block.
 * @returns {void}
 */
async function setIntervalWorksWithAsynchronousWait() {
    setInterval(() => console.info("hello"), 1000);
    console.info("Going to sleep");
    await sleep(3000);
    console.info("Back from sleep");
}

/**
 * What happens when you call `setImmediate()` with `await`. It pushes everything below it to execute in the end of the
 * current event loop iteration.
 * @returns {void}
 */
async function immediateWithPromises() {
    console.info("1");
    // setImmediate will push 2 to appear after 3
    setImmediate(() => { console.info("2"); });
    console.info("3");
    // awaiting on setImmediate will make 5 be postponed as well, so 4 will end up showing before 5
    await new Promise(resolve => setImmediate(() => { console.info("4"); resolve(); }));
    console.info("5");
}

setIntervalDoesNotWorkWithBlockingWait();
setIntervalWorksWithAsynchronousWait();
// immediateWithPromises();  // <-- only works with Node.js because of `setImmediate()`