asyncTest.js

/**
 * Going to be testing 3 things:
 *  1. Function marked async, but that still runs sync.
 *  2. Function marked async and returns a promise, but that still runs sync.
 *  3. Function marked async and that runs async by using process.nextTick().
 * 
 *  Note: async and sync are defined in the context of the Node.js event loop,
 *  which technically runs everything on one thread.
 *  
 *  Usage: this is a vanillaJS node script. So all you need to to do is type
 *  "node asyncTest.js" to run this.
 *
 */

/**
 * Test 1
 * 
 * Function looks async, but actually isn't.
 * Marking something async doesn't make it run asynchronously.
 * 
 */
 async function supposedToBeAsync_Test1() {
    wait();
    finish();
 }

/**
 * Test 2
 * 
 * Async function does not behave correctly just by using a promise.
 * Returning a Promise doesn't make it run asynchronously.
 * 
 */
async function supposedToBeAyncWithPromise_Test2() {
    supposedToBeAyncWithPromiseHelper();
    finish();
}

async function supposedToBeAyncWithPromiseHelper() {
    return new Promise((resolve, reject) => {
        wait(10000);
        resolve();
    });
}

/**
 * Test 3
 * 
 * Async with process.nextTick().
 * Runs the wait() call in the next cycle of the event loop.
 * 
 */
async function asyncWithProcessNextTick_Test3() {
    asyncWithProcessNextTickHelper();
    finish();
}

async function asyncWithProcessNextTickHelper() {
    process.nextTick(() => {
        wait(100);
    });
}

async function wait(len = 5) {
    console.log('Begin - wait()');
    let i = 0;
    while(i < len) {
        console.log(i);
        i++;
    }
    console.log('End - wait()');
 }

async function finish() {
    console.log('finish()');
}

// #### Uncomment to run test 1 ####
// supposedToBeAsync_Test1();
/**
 * Output of test 1 should be:
 -----------------------------
 Begin - wait()
 0
 1
 ...
 4
 End - wait()
 finish()
 -----------------------------

 Notice how "finish()" is printed last, which 
 shouldn't happen if "wait()" is async

 */

// #### Uncomment to run test 2 ####
// supposedToBeAyncWithPromise_Test2();
 /**
  * Output of test 2 should be:
 -----------------------------
 Begin - wait()
 0
 1
 ...
 9999
 End - wait()
 finish()
 -----------------------------

 Notice how "finish()" is printed last, which 
 shouldn't happen if "wait()" is async

 */

 // #### Uncomment to run test 3 ####
// asyncWithProcessNextTick_Test3();
 /**
  * Output of test 3 should be:
 -----------------------------
 finish()
 Begin - wait()
 0
 1
 ...
 99
 End - wait()
 -----------------------------

 Notice how "finish()" is printed first, which 
 is correct since "wait()" is correctly used async
 with the call to process.nextTick()

 */