Question

I have found some code from Underscore.js

  _.map = _.collect = function(obj, iterator, context) {
    var results = [];
    if (obj == null) return results;
    if (nativeMap && obj.map === nativeMap) return obj.map(iterator, context);
    each(obj, function(value, index, list) {
      results[results.length] = iterator.call(context, value, index, list);
    });
    if (obj.length === +obj.length) results.length = obj.length;
    return results;
  };

I would like to know what if (obj.length === +obj.length) does?

Answer 1

+length is a method to convert anything to a number.

If it’s a number, the value doesn’t change, and the comparison returns true.
If it’s not a number, the assertion is false.