5 ms·

What does `return function *(){…}` mean in JavaScript?

44 points by afshinmeh 13y ago

bdfh42 13y ago
But jump to this http://wingolog.org/archives/2013/05/08/generators-in-v8 http://wingolog.org/archives/2013/05/08/generators-in-v8 which explains it properly - and tells you why you would want to do it.
- afshinmeh 13y ago
  agreed
- k__ 13y ago
  Does it need to be used with promises?
  - apaprocki 13y ago
    No. Promises work nicely with generators, but you can use generator functions freely.
    - k__ 13y ago
      Is there any promise-less guide, that helps to grasp the concept of function* and yield in JS?
      - Touche 13y ago
        Not an article, but this is a good library that uses generator functions without promises: https://github.com/visionmedia/co https://github.com/visionmedia/co
      - masklinn 13y ago
        Any generator guide for Python will work, the syntax is slightly different (no function/def, the generator is "created" by the presence of a `yield` keyword in the body, and `yield*` is called `yield from`)
        k__ 13y ago
        The easiest thing to do is probably playing around with it :)
TillE 13y ago
Am I missing something, or does Stack Overflow really have this marked as a duplicate without actually linking to the earlier question? That's not very helpful.
- antsar 13y ago
  Not sure if it was there when you looked, but I see a link[1] to the relevant question in a grey box at the top of the question. I find the placement to be awkward though, I expected the link to be within the same box as the "marked as duplicate" text. [1] http://stackoverflow.com/questions/9620586/function-in-javascript http://stackoverflow.com/questions/9620586/function-in-javas...
  - smackfu 13y ago
    It doesn't stand out very much in this question because it has a big block of code which has the same styling as the alert box. When your interface design has so few colors, this kind of stuff happens.
  - cruise02 13y ago
    That's inserted automatically when questions are closed as duplicate.
- deleted 13y ago
  [deleted]
mhitza 13y ago
The asterix after the function keyword smells like a syntax hack. Firefox did it right, https://developer.mozilla.org/en-US/docs/Web/JavaScript/New_in_JavaScript/1.7 https://developer.mozilla.org/en-US/docs/Web/JavaScript/New_...
- masklinn 13y ago
  Depends what you mean by "syntax hack". Its point is to create a syntactic scope where `yield` can not currently exist and can be reserved. Since yield is a valid name in javascript today there might be code in the wild going function foo() { yield = some_computation(); return yield; } this code would be broken by enabling generators. Same with yield* foo() (which would be interpreted as multiplying the variable `yield` by the result of `foo()` in ES5). It also creates a nice parallelism between function* and yield, as a side-effect. See http://esdiscuss.org/topic/proposal-to-simplify-generators-impact#content-1 http://esdiscuss.org/topic/proposal-to-simplify-generators-i..., there should be more extensive previous discussions in es-discuss and TC39 notes but it's a pain in the ass to look it up. edit: https://mail.mozilla.org/pipermail/es-discuss/ https://mail.mozilla.org/pipermail/es-discuss/ also points to the use case of 0-item generators — mostly in relation to delegating from yield — with function* () {} in Python (and JS/1.7) you'd need the unwieldy function() { if (false) yield } and notes that having a star in the header function makes it easier on the reader.
  - deleted 13y ago
    [deleted]
  - ynniv 13y ago
    Why would yield as an lvalue or var conflict with yield as a statement? Yes it would be unavoidable if a variable named yield was used as a statement, but that's a pretty strange thing to do. Reserved words are unnecessary. As to convenient empty generators, they don't need to be enabled by syntax. You could define a function empty_gen() that is even fewer characters than function* () {}.
    - 13y ago

44 points by afshinmeh 13y ago

But jump to this http://wingolog.org/archives/2013/05/08/generators-in-v8 http://wingolog.org/archives/2013/05/08/generators-in-v8 which explains it properly - and tells you why you would want to do it.

afshinmeh 13y ago
agreed
k__ 13y ago
Does it need to be used with promises?
- apaprocki 13y ago
  No. Promises work nicely with generators, but you can use generator functions freely.
  - k__ 13y ago
    Is there any promise-less guide, that helps to grasp the concept of function* and yield in JS?
    - Touche 13y ago
      Not an article, but this is a good library that uses generator functions without promises: https://github.com/visionmedia/co https://github.com/visionmedia/co
    - masklinn 13y ago
      Any generator guide for Python will work, the syntax is slightly different (no function/def, the generator is "created" by the presence of a `yield` keyword in the body, and `yield*` is called `yield from`)
      - k__ 13y ago
        The easiest thing to do is probably playing around with it :)

TillE 13y ago

Am I missing something, or does Stack Overflow really have this marked as a duplicate without actually linking to the earlier question? That's not very helpful.

antsar 13y ago
Not sure if it was there when you looked, but I see a link[1] to the relevant question in a grey box at the top of the question. I find the placement to be awkward though, I expected the link to be within the same box as the "marked as duplicate" text. [1] http://stackoverflow.com/questions/9620586/function-in-javascript http://stackoverflow.com/questions/9620586/function-in-javas...
- smackfu 13y ago
  It doesn't stand out very much in this question because it has a big block of code which has the same styling as the alert box. When your interface design has so few colors, this kind of stuff happens.
- cruise02 13y ago
  That's inserted automatically when questions are closed as duplicate.
deleted 13y ago
[deleted]

mhitza 13y ago

The asterix after the function keyword smells like a syntax hack. Firefox did it right, https://developer.mozilla.org/en-US/docs/Web/JavaScript/New_in_JavaScript/1.7 https://developer.mozilla.org/en-US/docs/Web/JavaScript/New_...

masklinn 13y ago
Depends what you mean by "syntax hack". Its point is to create a syntactic scope where `yield` can not currently exist and can be reserved. Since yield is a valid name in javascript today there might be code in the wild going function foo() { yield = some_computation(); return yield; } this code would be broken by enabling generators. Same with yield* foo() (which would be interpreted as multiplying the variable `yield` by the result of `foo()` in ES5). It also creates a nice parallelism between function* and yield, as a side-effect. See http://esdiscuss.org/topic/proposal-to-simplify-generators-impact#content-1 http://esdiscuss.org/topic/proposal-to-simplify-generators-i..., there should be more extensive previous discussions in es-discuss and TC39 notes but it's a pain in the ass to look it up. edit: https://mail.mozilla.org/pipermail/es-discuss/ https://mail.mozilla.org/pipermail/es-discuss/ also points to the use case of 0-item generators — mostly in relation to delegating from yield — with function* () {} in Python (and JS/1.7) you'd need the unwieldy function() { if (false) yield } and notes that having a star in the header function makes it easier on the reader.
- deleted 13y ago
  [deleted]
- ynniv 13y ago
  Why would yield as an lvalue or var conflict with yield as a statement? Yes it would be unavoidable if a variable named yield was used as a statement, but that's a pretty strange thing to do. Reserved words are unnecessary. As to convenient empty generators, they don't need to be enabled by syntax. You could define a function empty_gen() that is even fewer characters than function* () {}.
  - 13y ago