/js-context-this

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JavaScript: Context & this

What is this?

Objectives

By the end of this lesson, students should be able to:

  • Recall whether or not this is determined at declaration.
  • Explain what this points to in each calling context.
  • Read and follow the execution context of code that uses different this idioms.

Preparation

  1. Fork and clone this repository.

this Is A Reference

We use this similar to the way we use pronouns in natural languages like English and French. We write: “John is running fast because he is trying to catch the train.” Note the use of the pronoun “he.” We could have written this: “John is running fast because John is trying to catch the train.” We don’t reuse “John” in this manner, for if we do, our family, friends, and colleagues would abandon us. Yes, they would. In a similar aesthetic manner, we use the this keyword as a shortcut, a referent to refer to an object.

Source: Understanding Javascript 'this' pointer.

this Changes by Call Context

A function can indiscriminately operate upon any object. When a function is invoked, it is bound to an object on which it operates. The contextual object on which a function operates is referenced using the keyword this.

let xwing = {
    pilot: null,

    setPilot: function(pilot) {
        this.pilot = pilot;
        this.update();
    },

    update: function() {
        console.log('This X-Wing has changed!');
    }
};

xwing.setPilot("Luke Skywalker");
// >> "This X-Wing has changed!"

console.log(xwing.pilot);
// >> "Luke Skywalker"

The Four Patterns of Invocation

We must invoke a function to run it (ie: call upon the function to do its thing). Amazingly, there are FOUR ways to invoke a function in JavaScript. This makes JS both amazingly flexible and absolutely insane.

Function Invocation Pattern

When a function is invoked without context, the function is bound to global scope:

function goBoom() {
    console.log(this);
}

goBoom();
// this === window

// This is the same as:
window.goBoom();

Context: this refers to the window object (global scope). Here we would say "a method is called on an object". In this case the object is the window.

Gotcha: This behavior has changed in ECMAScript 5 only when using strict mode: 'use strict';

Method Invocation Pattern

When a function is defined on an object, it is said to be a method of the object. When a method is invoked through its host object, the method is bound to its host:

let deathstar = {
    goBoom: function() {
      console.log(this);
  }
};

deathstar.goBoom();
// this === deathstar

Context: this refers to the host object.

Call/Apply Invocation Pattern

Function objects have their own set of native methods, most notably are .call and .apply. These methods will invoke the function with a provided contextual object.

function goBoom() {
    console.log(this);
}

let deathstar = {};
goBoom.call(deathstar);
// this === deathstar

Context: this refers to the passed object. Here you would say "the object receives the method".

Constructor Invocation Pattern

Any function may act as a constructor for new object instances. New object instances may be constructed with the "new" keyword while invoking a function.

Constructors are very similar to Ruby class constructors, in that they represent proper nouns within our application. Therefore they should follow the convention of capitalized names:

function Deathstar() {
    console.log(this);
}

let deathstar = new Deathstar();
// this === shiny new Deathstar instance

Context: this refers to the newly-created object instance. Here we would say "the object receives the method".

How this breaks down:

  1. Creates a new empty object ({})
  2. Attaches the constructor to the object as a property
  3. Invokes the constructor function on the new object
  4. Returns the object

Summary

  1. Is the function called with new (new binding)? If so, this is the newly constructed object. let bar = new foo()
  2. Is the function called with call or apply (explicit binding), even hidden inside a bind hard binding? If so, this is the explicitly specified object. let bar = foo.call( obj2 )
  3. Is the function called with a context (implicit binding), otherwise known as an owning or containing object? If so, this is that context object. let bar = obj1.foo()
  4. Otherwise, default the this (default binding). If in strict mode, pick undefined, otherwise pick the global object. let bar = foo()

Source: You-Dont-Know-JS/ch2.md

This and Array Methods

function Counter() {
  this.sum = 0;
  this.count = 0;
}
Counter.prototype.add = function(array) {
  array.forEach(function(entry) {
    this.sum += entry;
    ++this.count;
  }, this);
  // ^---- Note
};

let obj = new Counter();
obj.add([2, 5, 9]);
obj.count
// 3
obj.sum
// 16

Since obj.add() calls add() with this referring to obj, in add passing this into forEach() makes this in the forEach() callback also refer to obj.

forEach - JavaScript | MDN

Binding

Consider the following code:

//            <button>Get Random Person</button>​
​//        <input type="text">​
​
​
​var user = {
    data        :[
        {name:"T. Woods", age:37},
        {name:"P. Mickelson", age:43}
    ],
    clickHandler:function (event) {
        var randomNum = ((Math.random () * 2 | 0) + 1) - 1; // random number between 0 and 1​
​
        // This line is adding a random person from the data array to the text field​
        $ ("input").val (this.data[randomNum].name + " " + this.data[randomNum].age);
    }
​
}
​
​// Assign an eventHandler to the button's click event​
$ ("button").click (user.clickHandler);

What is happening and will this work?

With the .bind() method we can bind the context of user.clickHandler to the user object like so:

$ ("button").click (user.clickHandler.bind (user));

Extra: Fat Arrow (New in ES6)

Consider the following code, what is this?

$('.current-time').each(function () {
  setInterval(function () {
    $(this).text(Date.now());
  }, 1000);
});

The above code won't return do what you want, can you think of a way to get the code to do what is expcted?

Figure this out on your own.

Now with arrow functions (commonly referred to as "fat arrow"), you could write the code below and it would have the intended effect:

$('.current-time').each(function () {
  setInterval(() => $(this).text(Date.now()), 1000);
});

Fat arrow quick takes:

  1. It does not create it's own this context.
  2. A one line fat arrow function has an implicit return.

Lab (Pair)

Pair with a partner and follow the instructions in this.html. Your goal in this assignment is to read and understand the code examples presented. Take time to contemplate the execution flow, and note any questions you have for discussion.

Many of these scripts use the special debugger keyword to stop JS execution and open your console. Use this opportunity to inspect your environment (perhaps by looking at this?) and then continue.

Additional Resources

License

Source code distributed under the MIT license. Text and other assets copyright General Assembly, Inc., all rights reserved.