What is the importance of breaker in underscore.js

问题:

This question already has an answer here:

• underscore's each checking for {} return of callback 1 answer

In underscore.js, there is code like this:

if (iterator.call(context, obj[keys[i]], keys[i], obj) === breaker) return;

for example in "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, 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;
    }
  }
};

I don't understand, why add this if(xxx === breaker) return;? When will iterator.call(context, obj[keys[i]], keys[i], obj) === breaker?

回答1:

From the Annotated source code,

Establish the object that gets returned to break out of a loop iteration.

It is just a sentinel value and if the iterator function returns breaker, _.each will return immediately. Since, breaker is not assigned to any other object, no other object can be equal to breaker (it is matched with ===). It is used internally, by _.every and _.any.

_.every

if (!(result = result && iterator.call(context, value, index, list))) return breaker;

_.any

if (result || (result = iterator.call(context, value, index, list))) return breaker;

These two functions return breaker, so that _.each will break out of the iterations.

回答2:

It is used in other _underscore helpers, like _.all(), _.every(), _.any() which use the _.each() function to loop through the collection, but return the breaker when they wanna stop the each loop (i.e. they found the element they were looking for etc.)

回答3:

If you scroll up in the code, you will see the following code:

var breaker = {};

So it is checking if the function it is calling (the iterator) is returning an empty object. If it is, the each "loop" is broken and the function ends.