loop variable declaration style in for loop

Question

var each = _.each = _.forEach = function(obj, iterator, context) {
    if (obj == null) return obj;
    if (nativeForEach && obj.forEach === nativeForEach) {
      obj.forEach(iterator, context);
    } else if (obj.length === +obj.length) {
      for (var i = 0, length = obj.length; i < length; i++) {
        if (iterator.call(context, obj[i], i, obj) === breaker) return;
      }
    } else {
      var keys = _.keys(obj);
      for (var i = 0, length = keys.length; i < length; i++) {
        if (iterator.call(context, obj[keys[i]], keys[i], obj) === breaker) return;
      }
    }
    return obj;
  };

In the implementation of _.each method in underscore.js, the loop variables are set as follows.

for (var i = 0, length = obj.length; i < length; i++)

why is the length variable explicitly declared?, isn't the following code more succinct?

for (var i = 0; i < obj.length; i++)

It's more succinct but less efficient. In the second loop you're extracting obj.length on every iteration. In the first, you're just checking against a set value. — user2136840
– user2136840, Commented Feb 21, 2014 at 7:05

jfriend00 · Accepted Answer · 2014-02-21 07:05:47Z

2

There are a couple possible reasons to declare the separate length variable:

Caching it locally in its own variable can be faster than referencing it as a property every time through the loop.
In some cases, the array length may grow during the iteration and you may not want to iterate newly added elements.

answered Feb 21, 2014 at 7:05

jfriend00

711k104 gold badges1.1k silver badges1k bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

loop variable declaration style in for loop

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related