CODEWITHSUNDEEP
javascriptunderscore.js
Chris Muench
Chris Muench

Reputation: 18318

_.each context variable

I don't understand the purpose of a context variable for many of the underscore.js functions. What purpose does it serve. I know that it binds "this" in the iterator callback, but I don't understand the practical application of it.

var context = {'a': 'a'};
_.each([1, 2, 3], function(element, index, list)
{
    console.log(this);
    console.log(element);
    console.log(index);
    console.log(list);
}, context);

Upvotes: 3

Views: 1394

Answers (4)

Ashish Rana
Ashish Rana

Reputation: 85

As of version there are about 21 functions in underscore that accept 'context' as last optional argument.

_.each(list, iteratee(element, index, list), [context])

Iterates over a list of elements, yielding each in turn to an iteratee function. The iteratee is bound to the context object, if one is passed.

var array_1 = ['asdf', 'ghjk', 'lzxc', 'vbnm', 'qwer', 'tyui', 'op'];
var array_2 = [1,0,2,9,3,8,4,7,5,6];

_.each(array_1, function(element, index, list){
    console.log(this.length - index + " - " + this[this.length - index]);
}, array_2);

will output

1 - asdf
0 - ghjk
2 - lzxc
9 - vbnm
3 - qwer
8 - tyui
4 - op

here this is corresponds to the context array array_2.

Upvotes: 0

Flops
Flops

Reputation: 1420

It's realy helpfull when you dont need a context change, don't forget about it's hard dependecy brother Backbone.

var Collection = Backbone.Collection.extend({
//..
    _toggleActive: function (model, state) {
        model.set({
            active: state
        });
    },
    deactivateAll: function () {
        // analog _.each(this.models , func, [context])
        this.each(function (model) {
            // call the method of collection from context
            this._toggleActive(model, false);
        }, this);
    }
//..
});

Or just for debug 

_.each([1,2,3], function(item, i, arr){
    this.log(item, i);
}, console);

Upvotes: 0

tow
tow

Reputation: 569

It's useful if your iterator function is something like a method on an object:

var context = {'a': 'a',  foo: function(x) { console.log( this.a + x); }};
_.each([1, 2, 3], context.foo, context);

Upvotes: 2

Interrobang
Interrobang

Reputation: 17434

Underscore's _.each looks like this:

_.each(list, iterator, [context])

Contexts are very useful when your iterator is a member of some object you've created, and you want to execute that function in the scope of the object and not of the window. If your pre-written function that you're using as an iterator uses this to refer to an instance of an Object (as is generally the case), calling the function without context will result in this referring to the wrong thing.

Upvotes: 2

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