1. Concurrency models#

  • Processes
  • Threads (system or green)
  • Futures
    • Asynchronous operations, non-blocking, single-threaded
  • Coroutines
  • CSP
  • Actor

Learn more: java - What’s the difference between a Future and a Promise? - Stack Overflow

2. Promise object#

  • A wrapper for a resolved value or a reason that it’s not resolved yet.
  • But with some powerful methods, allow you handle different situations.
  • Like the then chain, the catch
  • Non-blocking: enable you to write asynchronous code in a synchronous manner.
    • Most important feature.
  • There are more complicated methods,

The Promise object has several fields and methods, including:

  1. state: A private field that represents the current state of the promise (pending, fulfilled or rejected).
  2. result: A private field that holds the result value if the promise is fulfilled or the reason if it is rejected.
  3. then(), catch(), finally() methods, only get executed when the state of Promise object is fulfilled or rejected.

3. Use await/async with Promise object# 

async function main() {
    let response
    try {
        // 'response' is not a Promise object, 
        // it's the resolved value the in the Promise object
        response = await fetch('https://www.google.com');
    } catch (err) {
        console.error("an err happened");
        return
    }
    console.log(response.ok);
}

main().then()

You may wonder why we need call then() after main(), the reason is that async functions always return a promise implicitly.

If the return value of an async function is not explicitly a promise, it will be implicitly wrapped in a promise. An async function is really just a fancy Promise wrapper.

If you call main like this:

...

await main();

There will be an error:

await main();
^^^^^

SyntaxError: await is only valid in async functions and the top level bodies of modules

This is another syntax to call async function:

(async () => {
  await main()
})()