What is “function*” in JavaScript?

In this page I found a new JavaScript function type:

// NOTE: "function*" is not supported yet in Firefox.
// Remove the asterisk in order for this code to work in Firefox 13 

function* fibonacci() { // !!! this is the interesting line !!!
    let [prev, curr] = [0, 1];
    for (;;) {
        [prev, curr] = [curr, prev + curr];
        yield curr;

I already know what yield, let and [?,?]=[?,?] do, but have no idea what the function* is meant to be. What is it?

  • ES6 Promise replacement of async.eachLimit / async.mapLimit
  • How to nest template strings in ES6?
  • linguistic meaning of 'let' variable in programming
  • Best way to implement logout in Firebase v3.0.1? Firebase.unauth is removed after update
  • ES6 promise settled callback?
  • ECMAScript 6 documentation generators
  • P.S. don’t bother trying Google, it’s impossible to search for expressions with asterisks (they’re used as placeholders).

  • import statement returns module member as undefined
  • How to destructure object properties with key names that are invalid variable names?
  • Has anyone used ECMAScript Proxies yet?
  • Why does const work in some for-loops in JavaScript?
  • Array intersection (set-theoretic) with Array.prototype.reduce
  • What's the difference between using “let” and “var” to declare a variable?
  • 3 Solutions collect form web for “What is “function*” in JavaScript?”

    It’s a proposed syntax for EcmaScript.next.

    Dave Herman of Mozilla gave a talk about EcmaScript.next. At 30:15 he talks about generators.

    Earlier, he explains how Mozilla is experimentally implementing proposed language changes to help steer the committee. Dave works closely with Brendan Eich, Mozilla’s CTO (I think), and the original JavaScript designer.

    You can find more detail on the EcmaScript working group wiki: http://wiki.ecmascript.org/doku.php?id=harmony:generators

    The working group (TC-39) has general agreement that EcmaScript.next should have some kind of generator iterator proposal, but this is not final.

    You shouldn’t rely on this showing up without changes in the next version of the language, and even if it doesn’t change, it probably won’t show up widely in other browsers for a while.


    First-class coroutines, represented as objects encapsulating suspended execution contexts (i.e., function activations). Prior art: Python, Icon, Lua, Scheme, Smalltalk.


    The “infinite” sequence of Fibonacci numbers (notwithstanding behavior around 253):

    function* fibonacci() {
        let [prev, curr] = [0, 1];
        for (;;) {
            [prev, curr] = [curr, prev + curr];
            yield curr;

    Generators can be iterated over in loops:

    for (n of fibonacci()) {
        // truncate the sequence at 1000
        if (n > 1000)

    Generators are iterators:

    let seq = fibonacci();
    print(seq.next()); // 1
    print(seq.next()); // 2
    print(seq.next()); // 3
    print(seq.next()); // 5
    print(seq.next()); // 8

    It’s a generator function – and it said so in the page you cite, in the comment you replaced with “this is the interesting line”…

    Basically it’s a way to specify sequences programmatically so that they can be passed around and elements accessed by index without having to compute the entire sequence (possibly infinite in size) beforehand.

    The function* type looks like it acts as a generator function for processes that can be iterated.
    C# has a feature like this using “yield return” see 1 and see 2

    Essentially this returns each value one by one to whatever is iterating this function, which is why their use case shows it in a foreach style loop.