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2

I have seen anonymous functions inside for loops to induce new scope on the web in one or two places and would like to know if it makes sense.

for example:

var attr, colors = ['green','blue','red'];

for ( attr = 0; attr < colors.length; attr++) {
    (function() {
        var colorAttr = colors[attr];

        // do something with colorAttr
    })();
}

I understand it has something to do with keeping the scope inside the for loop clean, but in what situations would this be necessary? Would it be good practice to do this everywhere you need to declare a new var inside the for loop?

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Here is an example where closure in loops make sense: stackoverflow.com/questions/1552941/… –  Salman A Dec 20 '12 at 17:05
1  
attr changes as the loop progresses, so if you use colors[attr] in a callback function, it won't refer to the colors[attr] that you actually want. –  Blender Dec 20 '12 at 17:05
 
I use async calls only when waiting for something to complete. In this case, it may be problematic to track the colors[] changes. –  mkey Dec 20 '12 at 17:07
 
so does a pattern like this make more sense if you are attaching an event handler inside? –  Evan Dec 20 '12 at 17:08
 
This isn't a callback - it is an immediately executing function. colorAttr is scoped to that immediately executing function, but it can walk up to the parent scope and happily access attr. –  Steve Fenton Dec 20 '12 at 17:09
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2 Answers

up vote 5 down vote accepted

When you have inner functions that are not executed immediately, as part of the loop.

var i, colors = ['green','blue','red'];
for ( i = 0; i < colors.length; i++) {
  var color = colors[i];
  setTimeout(function() {
    alert(color);
  }, i * 1000);
}

// red
// red
// red

Even though var color is inside the loop, loops have no scope. You actually only have one variable that every loop iteration uses. So when the timeouts fire, they all use the same value, the last value set by the loop.

var i, colors = ['green','blue','red'];
for ( i = 0; i < colors.length; i++) {
  (function(color) {
    setTimeout(function() {
      alert(color);
    }, i * 1000);
  )(colors[i]);
}

// green
// blue
// red

This one captures the value at each iteration into an argument to a function, which does create a scope. Now each function gets it's own version of a color variable which won't change when functions created within that loop are later executed.

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ohhhhhhhhhh that makes sense –  Evan Dec 20 '12 at 17:10
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up vote 1 down vote

You're almost there. It makes only sense in your snippet, if you pass in the attr value into your self invoking function as an argument. That way, it can store that variable within its very own scope object

(function( attr ) {
    var colorAttr = colors[attr];

    // do something with colorAttr
})( attr );

Now, the activation object respectively lexical environment record (which are ES3 and ES5 scope objects) will have an entry for whatever value is behind attr and therefore, its closured.

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