returning a function that uses a var declared in the parent function

Question

this is a function that chooses items randomly from a given array until all are taken and only then repeating

i actually copied the code from a post because i couldn't understand how the variable copy's scope works .

PS : here's the post How to efficiently randomly select array item without repeats?

thanks in advance for the help .

function randomNoRepeats(array) {
  var copy = array.slice(0);
  return function() {
    if (copy.length < 1) { copy = array.slice(0); }
    var index = Math.floor(Math.random() * copy.length);
    var item = copy[index];
    copy.splice(index, 1);
    return item;
  };
}

var chooser = randomNoRepeats(['Foo', 'Bar', 'Gah']);
chooser(); // => "Bar"
chooser(); // => "Foo"
chooser(); // => "Gah"
chooser(); // => "Foo" -- only repeats once all items are exhausted.

What exactly is your question? How `chooser` is able to retain `copy`? — , Dec 01 '18 at 13:09
calling the `randomNoRepeats`, returns you a new function that can then be called, as long as that function is alive `array` & `copy` are alive in the closure. The most interesting that can happen (after all words have past once), that if the `array` sent in changes (as it is a reference), that the function will also adapt to those changes upon the next reset ^_^ — Icepickle, Dec 01 '18 at 13:10
The inner function is [known as a `closure`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Closures) and it "closes" around the outer lexical variables and takes them with it when it's returned. — Andy, Dec 01 '18 at 13:10

score 2 · Accepted Answer · answered Dec 01 '18 at 13:16

2

This is an example of [closures] in JavaScript.

According to MDN, functions in JavaScript form closures. A closure is the combination of a function and the lexical environment within which that function was declared. This environment consists of any local variables that were in-scope at the time the closure was created.

In your case, chooser is a reference to an instance of the function which is returned inside the function randomNoRepeats when the latter is called. The instance of this returned function maintains a reference to its lexical environment, within which the variable copy exists. For this reason, when chooser is invoked, the variable copy remains available for use.

returning a function that uses a var declared in the parent function

1 Answers1