Question

Lastly I was skimming through Underscore.js code to learn JavaScript idioms and I found following definition of each function:

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

What this line is for? (I assume that this is a way of checking if passed object is an array, am I right? If so, wouldn’t be an typeof operator better approach?)

obj.length === +obj.length

Answer 1

The each method accepts Collections (Arrays or Objects).

After checking if the native method forEach is available, the method checks if obj is an Array. This is not possible using typeof since typeof will return "object" for arrays as well for objects (try typeof [] === typeof {}). So, they used the obj.length === +obj.length approach.

This works because if obj doesn’t have the length property, +obj.length returns NaN and the strict comparison fails. Then, the method assumes obj is an object.