What does a + prefix do in this context?

Question

Here is a line of code from underscore. What is that plus prefix for in this line?

if (obj.length === +obj.length) { // plus prefix?

Answer 1

This forces the value of obj.length to be a Number. Essentially this is done to make sure that the default length value for an array-like object has not been overridden so that it can be iterated properly.

breaker will do nothing in this context because even another empty object {} will evaluate to false when compared to breaker .. even without an equivalence comparison.

However, breaker is not used in that context because it is defined outside of the .each function, which appears different than what you are showing here. Instead, it is used to force a "break" from other looping methods:

_.every = _.all = function(obj, iterator, context) {
  /* snip */
  if (!(result = result && iterator.call(context, value, index, list)))
      return breaker;

You can see that if the result is not truthy in "every," we want to break immediately. _.every calls _.each, and it will return breaker which will be true when compared to itself, allowing for an immediate break.