You Don't Know JS: Async & Performance

Appendix B. Advanced Async Patterns

Appendix A introduced the asynquence library for sequence-oriented async flow control, primarily based on Promises and generators.

Now we’ll explore other advanced asynchronous patterns built on top of that existing understanding and functionality, and see how asynquence makes those sophisticated async techniques easy to mix and match in our programs without needing lots of separate libraries.

Iterable Sequences

We introduced asynquence’s iterable sequences in the previous appendix, but we want to revisit them in more detail.

To refresh, recall:

var domready = ASQ.iterable();

// ..

domready.val( function(){
    // DOM is ready
} );

// ..

document.addEventListener( "DOMContentLoaded", domready.next );

Now, let’s define a sequence of multiple steps as an iterable sequence:

var steps = ASQ.iterable();

steps
.then( function STEP1(x){
    return x * 2;
} )
.steps( function STEP2(x){
    return x + 3;
} )
.steps( function STEP3(x){
    return x * 4;
} );

steps.next( 8 ).value;  // 16
steps.next( 16 ).value; // 19
steps.next( 19 ).value; // 76
steps.next().done;      // true

As you can see, an iterable sequence is a standard-compliant iterator (see Chapter 4). So, it can be iterated with an ES6 for..of loop, just like a generator (or any other iterable) can:

var steps = ASQ.iterable();

steps
.then( function STEP1(){ return 2; } )
.then( function STEP2(){ return 4; } )
.then( function STEP3(){ return 6; } )
.then( function STEP4(){ return 8; } )
.then( function STEP5(){ ...