What is context in _.each(list, iterator, [context

Question

I am new to underscore.js. What is the purpose of [context] in _.each()? How should it be used?

Answer 1

Simple use of _.each

_.each(['Hello', 'World!'], function(word){
    console.log(word);
});

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Here's simple example that could use _.each:

function basket() {
    this.items = [];
    this.addItem = function(item) {
        this.items.push(item);
    };
    this.show = function() {
        console.log('items: ', this.items);
    }
}

var x = new basket();
x.addItem('banana');
x.addItem('apple');
x.addItem('kiwi');
x.show();

Output:

items:  [ 'banana', 'apple', 'kiwi' ]

Instead of calling addItem multiple times you could use underscore this way:

_.each(['banana', 'apple', 'kiwi'], function(item) { x.addItem(item); });

which is identical to calling addItem three times sequentially with these items. Basically it iterates your array and for each item calls your anonymous callback function that calls x.addItem(item). The anonymous callback function is similar to addItem member function (e.g. it takes an item) and is kind of pointless. So, instead of going through anonymous function it's better that _.each avoids this indirection and calls addItem directly:

_.each(['banana', 'apple', 'kiwi'], x.addItem);

but this won't work, as inside basket's addItem member function this won't refer to your x basket that you created. That's why you have an option to pass your basket x to be used as [context]:

_.each(['banana', 'apple', 'kiwi'], x.addItem, x);

Full example that uses _.each and context:

function basket() {
    this.items = [];
    this.addItem = function(item) {
        this.items.push(item);
    };
    this.show = function() {
        console.log('items: ', this.items);
    }
}
var x = new basket();
_.each(['banana', 'apple', 'kiwi'], x.addItem, x);
x.show();

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In short, if callback function that you pass to _.each in any way uses this then you need to specify what this should be referring to inside your callback function. It may seem like x is redundant in my example, but x.addItem is just a function and could be totally unrelated to x or basket or any other object, for example:

function basket() {
    this.items = [];
    this.show = function() {
        console.log('items: ', this.items);
    }
}
function addItem(item) {
    this.items.push(item);
};

var x = new basket();
_.each(['banana', 'apple', 'kiwi'], addItem, x);
x.show();

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In other words, you bind some value to this inside your callback, or you may as well use bind directly like this:

_.each(['banana', 'apple', 'kiwi'], addItem.bind(x));

how this feature can be useful with some different underscore methods?

In general, if some underscorejs method takes a callback function and if you want that callback be called on some member function of some object (e.g. a function that uses this) then you may bind that function to some object or pass that object as the [context] parameter and that's the primary intention. And at the top of underscorejs documentation, that's exactly what they state: The iteratee is bound to the context object, if one is passed

Answer 2

context is where this refers to in your iterator function. For example:

var person = {};
person.friends = {
  name1: true,
  name2: false,
  name3: true,
  name4: true
};

_.each(['name4', 'name2'], function(name){
  // this refers to the friends property of the person object
  alert(this[name]);
}, person.friends);

Answer 3

The context parameter just sets the value of this in the iterator function.

var someOtherArray = ["name","patrick","d","w"];

_.each([1, 2, 3], function(num) { 
    // In here, "this" refers to the same Array as "someOtherArray"

    alert( this[num] ); // num is the value from the array being iterated
                        //    so this[num] gets the item at the "num" index of
                        //    someOtherArray.
}, someOtherArray);

Working Example: http://jsfiddle.net/a6Rx4/

It uses the number from each member of the Array being iterated to get the item at that index of someOtherArray, which is represented by this since we passed it as the context parameter.

If you do not set the context, then this will refer to the window object.

Answer 4

As explained in other answers, context is the this context to be used inside callback passed to each.

I'll explain this with the help of source code of relevant methods from underscore source code

The definition of _.each or _.forEach is as follows:

_.each = _.forEach = function(obj, iteratee, context) {
  iteratee = optimizeCb(iteratee, context);

  var i, length;
  if (isArrayLike(obj)) {
    for (i = 0, length = obj.length; i < length; i++) {
      iteratee(obj[i], i, obj);
    }
  } else {
    var keys = _.keys(obj);
    for (i = 0, length = keys.length; i < length; i++) {
      iteratee(obj[keys[i]], keys[i], obj);
    }
  }
  return obj;
};

Second statement is important to note here

iteratee = optimizeCb(iteratee, context);

Here, context is passed to another method optimizeCb and the returned function from it is then assigned to iteratee which is called later.

var optimizeCb = function(func, context, argCount) {
  if (context === void 0) return func;
  switch (argCount == null ? 3 : argCount) {
    case 1:
      return function(value) {
        return func.call(context, value);
      };
    case 2:
      return function(value, other) {
        return func.call(context, value, other);
      };
    case 3:
      return function(value, index, collection) {
        return func.call(context, value, index, collection);
      };
    case 4:
      return function(accumulator, value, index, collection) {
        return func.call(context, accumulator, value, index, collection);
      };
  }
  return function() {
    return func.apply(context, arguments);
  };
};

As can be seen from the above method definition of optimizeCb, if context is not passed then func is returned as it is. If context is passed, callback function is called as

func.call(context, other_parameters);
          ^^^^^^^

func is called with call() which is used to invoke a method by setting this context of it. So, when this is used inside func, it'll refer to context.

// Without `context`
_.each([1], function() {
  console.log(this instanceof Window);
});


// With `context` as `arr`
var arr = [1, 2, 3];
_.each([1], function() {
  console.log(this);
}, arr);

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You can consider context as the last optional parameter to forEach in JavaScript.

Answer 5

The context lets you provide arguments at call-time, allowing easy customization of generic pre-built helper functions.

some examples:

// stock footage:
function addTo(x){ "use strict"; return x + this; }
function pluck(x){ "use strict"; return x[this]; }
function lt(x){ "use strict"; return x < this; }

// production:
var r = [1,2,3,4,5,6,7,8,9];
var words = "a man a plan a canal panama".split(" ");

// filtering numbers:
_.filter(r, lt, 5); // elements less than 5
_.filter(r, lt, 3); // elements less than 3

// add 100 to the elements:
_.map(r, addTo, 100);

// encode eggy peggy:
_.map(words, addTo, "egg").join(" ");

// get length of words:
_.map(words, pluck, "length"); 

// find words starting with "e" or sooner:
_.filter(words, lt, "e"); 

// find all words with 3 or more chars:
_.filter(words, pluck, 2); 

Even from the limited examples, you can see how powerful an "extra argument" can be for creating re-usable code. Instead of making a different callback function for each situation, you can usually adapt a low-level helper. The goal is to have your custom logic bundling a verb and two nouns, with minimal boilerplate.

Admittedly, arrow functions have eliminated a lot of the "code golf" advantages of generic pure functions, but the semantic and consistency advantages remain.

I always add "use strict" to helpers to provide native [].map() compatibility when passing primitives. Otherwise, they are coerced into objects, which usually still works, but it's faster and safer to be type-specific.