JavaScript ES6 Promises – How JS Promises Work in Single-Threaded Environment

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Javascript is single threaded. What I understand from this is if JS i executing on line of code lets say a function. It cannot go to the next line unless that function has been removed from the stack.

With that being said, I do not understand, how promises let us execute other code blocks down the road, while the interpreter is still resolving the promise code.

I am a noob in JS so I am trying to understand the underlying implementation.

I saw an example PHP implementation of Promises and the idea is that PHP can fork a separate process in a different thread that sends back signal to the main thread when the async code has been executed.

Does it work the same way in Javascript ?

Thanks

Best Answer

Parallel or out of order or asynchronous execution is not really tied to number of threads. Think of it. A single thread can still do the switching and do n things at same time, in bits and pieces. Like a software based CPU.

It's really how the execution platform is implemented. You can have an illusion of such processing via an event queue and a single looper thread that works on that queue.

In case of JavaScript engines, you can think that for every async operation defined on any line in source code, the engine does not wait to complete that operation, rather, simply place it in the queue and cater to it from time to time, until it is completed.

The promised (or future) values contain nothing when the line is executed, and next line is evaluated. Only some time later the values "resolve" and everything depending on that value is also resolved, and so on, until everything is resolved and the queue is empty. Think of a graph being collapsed back to root as values become available and pending calculation complete.

Related Solutions

JavaScript Asynchronous Programming – How Callbacks Work

It doesn't. Just taking a callback or passing a callback doesn't mean it's asynchronous.

For example, the .forEach function takes a callback but is synchronous.

var available = false;
[1,2,3].forEach( function(){
    available = true;
});
//code here runs after the whole .forEach has run,
//so available === true here

The setTimeout takes a callback too and is asynchronous.

function myFunction( fn ) {
    setTimeout( function() {
        fn(1,2,3);
    }, 0 );
}

var available = false;
myFunction( function() {
    available = true;
});
//available is never true here

Hooking to any asynchronous event in Javascript always requires a callback but that doesn't mean calling functions or passing them around is always asynchronous.

Javascript – Is there something a `Promise` can do that `async` cannot

Some of the advantages of promises are that they make asynchronous operations a "first-class" entity. They can be transformed, they are composable, and their use also happens to reduce nesting and make handling of async code more analogous to sync code. To me, it is usually more readable to have a chain of promises than nested callbacks. Many modern languages have moved to promise-style async.

See: http://domenic.me/2012/10/14/youre-missing-the-point-of-promises/

Best Answer

Related Solutions

JavaScript Asynchronous Programming – How Callbacks Work

Javascript – Is there something a `Promise` can do that `async` cannot

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