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Javascript Generators: Understanding them

Javascript Generators: Understanding them


I'm pretty sure my understanding of generators is inherently broken. All online resources seem to conflict and it makes for an incredibly difficult and confusing learning experience.

From what I understand, the yield keyword enables a currently executing block of code to wait for a value instead of throwing remaining code to be executed inside a callback. So, as most tutorials have pointed out, you can use this:

(function *() {
     // Wait until users have be got and put into value of `results`
     var results = yield db.get("users");
     // And continue

Instead of:

db.get("user", function(results) {

Right, that's all well and good until I try to write my own generators. I've run into several hitches:

  • The first example code I above will not run because there is nothing to iterate over the generator, correct? Some higher being has to call the .next somewhere, right?
  • The entire API will have to be rewritten right down to the I/O calls to support generators, correct?
  • From what I gather, yield seems to stand for wait for the value most general use cases whereas in the implementation part (read: return value to/inside db.get) yield seems to stand for send this value back to currently waiting block to resume execution.

Take for example:

function *fn() {
    yield 1;
    yield "a";

var gen = fn();; // 1; // "a";

yield in that context is sending values back down instead of waiting for the results. In the first example above, it waits for the results from the db.get and the resumes execution instead of "returning" or sending back a value. If the db.get case is true, is this not inherently synchronous? I mean, isn't it exactly the same as:

(function() {
     //Wait for the results
    var results = fs.readFileSync("users.txt");
    // Use results

Unfortunately, if it's any way clear from this question (probably the only thing clear) is that I don't understand generators. Hopefully, I might get some insight here.

Problem courtesy of: AdrianCooney


TL;DR: the essence of generator is controlling the suspension of code execution.

For generator itself, you can refer to this.

To sum up, there is three components you should distinguish: 1. generator Function 2. generator 3. generated result

Generator function is simply the function with star in its head and (optional) yield in its body.

function *generator() {
  var i = 0;
  while (true) {
    if (i 

Generator function itself does not do anything but return a generator, in the case above, gen. No console output here because only after the returned generator's next method is called the body of generator function will run. Generator has several methods, of which the most important one is next. next runs the code and returns the generator result.

var ret =;
// Start!
// {value: 0, done: false}

ret above is generator result. It has two property: value, the value you yield in generator function, and done, a flag indicating whether the generator function return.

// {value: 1, done: false}
// {value: 2, done: false}
// {value: undefined, done: true}

At this point, no one will expect you to understand generator, at least not the async power of generator.

To put it simple, generator has two features:

  • one can choose to jump out of a function and let outer code to determine when to jump back into the function.
  • the control of asynchronous call can be done outside of your code

In code, yield jumps outside of function, and next(val) jumps back to the function and pass value back into the function. Outside code can handle asynchronous call and decide proper time to switch to your own code.

Look the sample again:

var gen = generator();
console.log('generated generator');
// mock long long processing
setTimeout(function() {
  console.log('Execute after timer fire');
}, 1000);
console.log('Execute after timer set');

/* result:
    generated generator
    Execute after timer set
    Execute after timer fire

See? The generator function itself does not handle callback. The outside code does.

The base is here. You can elaborate this code to support full asynchronousity while keeping generator function like sync one.

For example, suppose geturl is an asynchronous call that returns a promise object. you can write var html = yield getUrl(''); This jumps outside your code. And outside code will do stuff like:

var ret =;
ret.then(function (fetchedHTML) {
  // jumps back to your generator function
  // and assign fetchHTML to html in your code;

For more complete guide, refer to this. And repository like co, galaxy, suspend and etc.

Solution courtesy of: Herrington Darkholme


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This post first appeared on Node.js Recipes, please read the originial post: here

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Javascript Generators: Understanding them


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