javascript - Wait for async task to finish

Question

I'm interacting with a third-party JavaScript library where some function calls are asynchronous. Instead of working the asynchronous logic into my application, I preferred to write synchronous wrappers to those async calls. I know, I know, it's terrible design, but this is a demo project with very high chance of being rewritten entirely. I need something to show the team the concept, not really having to worry performance, yet.

Here's what I wanna do:

function sync_call(input) {
    var value;

    // Assume the async call always succeed
    async_call(input, function(result) {value = result;} );

    return value;
}

I tried the jQuery's deferred and promise but it seems to be aiming at the async design pattern. I want to use the synchronous pattern in my code.

Answer 1

This will never work, because the JS VM has moved on from that async_call and returned the value, which you haven't set yet.

Don't try to fight what is natural and built-in the language behaviour. You should use a callback technique or a promise.

function f(input, callback) {
    var value;

    // Assume the async call always succeed
    async_call(input, function(result) { callback(result) };

}

The other option is to use a promise, have a look at Q. This way you return a promise, and then you attach a then listener to it, which is basically the same as a callback. When the promise resolves, the then will trigger.