What does `return function *(){...}` mean?

Question

I saw something strange in Koa. It has some new function names (from https://github.com/koajs/koa/blob/master/examples/co.js#L10):

app.use(function(){
  return function *(){
    var paths = yield fs.readdir('docs');

    var files = yield paths.map(function(path){
      return fs.readFile('docs/' + path, 'utf8');
    });

    this.type = 'markdown';
    this.body = files.join('');
  }
});

What does return function *() mean? Can we declare a function with the name of * in JavaScript?

score 35 · Accepted Answer · answered Nov 07 '13 at 10:52

35

It means that the function returns an iterator (so it can be repeatedly called with .next() to yield more values.

check out http://wingolog.org/archives/2013/05/08/generators-in-v8 for more info

It's an ES6 construct, so at the moment you see it more in node rather than client side js

answered Nov 07 '13 at 10:52

benjaminbenben

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BTW, it's only availaible in unstable node for now (>=0.11) with the flag `--harmony-generators` – toasted_flakes Dec 19 '13 at 19:56

score 17 · Answer 2 · edited Nov 07 '13 at 10:59

17

Koa makes use of a new JavaScript feature called generators, and the * is the way to identify a generator in V8.

edited Nov 07 '13 at 10:59

Alexander

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answered Nov 07 '13 at 10:51

Luc Morin

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What does `return function *(){...}` mean?

2 Answers2