Question

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underscore.js _.each(list, iterator, [context]) what is context?

So in the context of this forEach function in underscore.js:

// The cornerstone, an `each` implementation, aka `forEach`.
// Handles objects with the built-in `forEach`, arrays, and raw objects.
// Delegates to **ECMAScript 5**'s native `forEach` if available.
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 (_.has(obj, key)) {
        if (iterator.call(context, obj[key], key, obj) === breaker) return;
      }
    }
  }
};

What is the parameter context and how is it used?

Answer 1

Sets the this value (the calling context) of the iterator function that was passed.

iterator.call(context, obj[i], i, obj);
      //         ^---right here

JavaScript’s .call and .apply methods let you invoke a function with the this value of the function you’re calling set to the first argument you provide.

So if I do…

var obj = {foo:"bar"};
func.call(obj);

…the value of this inside func will be the {foo:"bar"} object.

So if you provide that argument, underscore uses it as the first argument to .call when invoking the function you passed as seen above.