I have a constructor function which registers an event handler:

function MyConstructor(data, transport) {
    this.data = data;
    transport.on('data', function () {
        alert(this.data);
    });
}

// Mock transport object
var transport = {
    on: function(event, callback) {
        setTimeout(callback, 1000);
    }
};

// called as
var obj = new MyConstructor('foo', transport);

However, I'm not able to access the data property of the created object inside the callback. It looks like this does not refer to the object that was created, but to another one.

I also tried to use an object method instead of an anonymous function:

function MyConstructor(data, transport) {
    this.data = data;
    transport.on('data', this.alert);
}

MyConstructor.prototype.alert = function() {
    alert(this.name);
};

but it exhibits the same problems.

How can I access the correct object?

Solution 1:

What you should know about this

this (aka "the context") is a special keyword inside each function and its value only depends on how the function was called, not how/when/where it was defined. It is not affected by lexical scopes like other variables (except for arrow functions, see below). Here are some examples:

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

// normal function call
foo(); // `this` will refer to `window`

// as object method
var obj = {bar: foo};
obj.bar(); // `this` will refer to `obj`

// as constructor function
new foo(); // `this` will refer to an object that inherits from `foo.prototype`

To learn more about this, have a look at the MDN documentation.

How to refer to the correct this

Use arrow functions

ECMAScript 6 introduced arrow functions, which can be thought of as lambda functions. They don't have their own this binding. Instead, this is looked up in scope just like a normal variable. That means you don't have to call .bind. That's not the only special behavior they have, please refer to the MDN documentation for more information.

function MyConstructor(data, transport) {
    this.data = data;
    transport.on('data', () => alert(this.data));
}

Don't use this

You actually don't want to access this in particular, but the object it refers to. That's why an easy solution is to simply create a new variable that also refers to that object. The variable can have any name, but common ones are self and that.

function MyConstructor(data, transport) {
    this.data = data;
    var self = this;
    transport.on('data', function() {
        alert(self.data);
    });
}

Since self is a normal variable, it obeys lexical scope rules and is accessible inside the callback. This also has the advantage that you can access the this value of the callback itself.

Explicitly set this of the callback - part 1

It might look like you have no control over the value of this because its value is set automatically, but that is actually not the case.

Every function has the method .bind ^[docs], which returns a new function with this bound to a value. The function has exactly the same behavior as the one you called .bind on, only that this was set by you. No matter how or when that function is called, this will always refer to the passed value.

function MyConstructor(data, transport) {
    this.data = data;
    var boundFunction = (function() { // parenthesis are not necessary
        alert(this.data);             // but might improve readability
    }).bind(this); // <- here we are calling `.bind()` 
    transport.on('data', boundFunction);
}

In this case, we are binding the callback's this to the value of MyConstructor's this.

Note: When a binding context for jQuery, use jQuery.proxy ^[docs] instead. The reason to do this is so that you don't need to store the reference to the function when unbinding an event callback. jQuery handles that internally.

Set this of the callback - part 2

Some functions/methods which accept callbacks also accept a value to which the callback's this should refer to. This is basically the same as binding it yourself, but the function/method does it for you. Array#map ^[docs] is such a method. Its signature is:

array.map(callback[, thisArg])

The first argument is the callback and the second argument is the value this should refer to. Here is a contrived example:

var arr = [1, 2, 3];
var obj = {multiplier: 42};

var new_arr = arr.map(function(v) {
    return v * this.multiplier;
}, obj); // <- here we are passing `obj` as second argument

Note: Whether or not you can pass a value for this is usually mentioned in the documentation of that function/method. For example, jQuery's $.ajax method ^[docs] describes an option called context:

This object will be made the context of all Ajax-related callbacks.

Common problem: Using object methods as callbacks/event handlers

Another common manifestation of this problem is when an object method is used as callback/event handler. Functions are first-class citizens in JavaScript and the term "method" is just a colloquial term for a function that is a value of an object property. But that function doesn't have a specific link to its "containing" object.

Consider the following example:

function Foo() {
    this.data = 42,
    document.body.onclick = this.method;
}

Foo.prototype.method = function() {
    console.log(this.data);
};

The function this.method is assigned as click event handler, but if the document.body is clicked, the value logged will be undefined, because inside the event handler, this refers to the document.body, not the instance of Foo.
As already mentioned at the beginning, what this refers to depends on how the function is called, not how it is defined.
If the code was like the following, it might be more obvious that the function doesn't have an implicit reference to the object:

function method() {
    console.log(this.data);
}


function Foo() {
    this.data = 42,
    document.body.onclick = this.method;
}

Foo.prototype.method = method;

The solution is the same as mentioned above: If available, use .bind to explicitly bind this to a specific value

document.body.onclick = this.method.bind(this);

or explicitly call the function as a "method" of the object, by using an anonymous function as callback / event handler and assign the object (this) to another variable:

var self = this;
document.body.onclick = function() {
    self.method();
};

or use an arrow function:

document.body.onclick = () => this.method();

Solution 2:

Here are several ways to access the parent context inside a child context -

1. You can use the bind() function.
2. Store a reference to context/this inside another variable (see the below example).
3. Use ES6 Arrow functions.
4. Alter the code, function design, and architecture - for this you should have command over design patterns in JavaScript.

1. Use the bind() function

function MyConstructor(data, transport) {
    this.data = data;
    transport.on('data', ( function () {
        alert(this.data);
    }).bind(this) );
}
// Mock transport object
var transport = {
    on: function(event, callback) {
        setTimeout(callback, 1000);
    }
};
// called as
var obj = new MyConstructor('foo', transport);

If you are using Underscore.js - http://underscorejs.org/#bind

transport.on('data', _.bind(function () {
    alert(this.data);
}, this));

2. Store a reference to context/this inside another variable

function MyConstructor(data, transport) {
  var self = this;
  this.data = data;
  transport.on('data', function() {
    alert(self.data);
  });
}

3. Arrow function

function MyConstructor(data, transport) {
  this.data = data;
  transport.on('data', () => {
    alert(this.data);
  });
}

Solution 3:

It's all in the "magic" syntax of calling a method:

object.property();

When you get the property from the object and call it in one go, the object will be the context for the method. If you call the same method, but in separate steps, the context is the global scope (window) instead:

var f = object.property;
f();

When you get the reference of a method, it's no longer attached to the object. It's just a reference to a plain function. The same happens when you get the reference to use as a callback:

this.saveNextLevelData(this.setAll);

That's where you would bind the context to the function:

this.saveNextLevelData(this.setAll.bind(this));

If you are using jQuery you should use the $.proxy method instead, as bind is not supported in all browsers:

this.saveNextLevelData($.proxy(this.setAll, this));

Solution 4:

You should know about "this" Keyword.

As per my view you can implement "this" in three ways (Self/Arrow function/Bind Method)

A function's this keyword behaves a little differently in JavaScript compared to other languages.

It also has some differences between strict mode and non-strict mode.

In most cases, the value of this is determined by how a function is called.

It can't be set by assignment during execution, and it may be different each time the function is called.

ES5 introduced the bind() method to set the value of a function's this regardless of how it's called,

And ES2015 introduced arrow functions that don't provide their own this binding (it retains this value of the enclosing lexical context).

Method1: Self - Self is being used to maintain a reference to the original this even as the context is changing. It's a technique often used in event handlers (especially in closures).

Reference: this

function MyConstructor(data, transport) {
    this.data = data;
    var self = this;
    transport.on('data', function () {
        alert(self.data);
    });
}

Method2: Arrow function - An arrow function expression is a syntactically compact alternative to a regular function expression, although without its own bindings to the this, arguments, super, or new.target keywords.

Arrow function expressions are ill-suited as methods, and they cannot be used as constructors.

Reference: Arrow function expressions

  function MyConstructor(data, transport) {
    this.data = data;
    transport.on('data',()=> {
        alert(this.data);
    });
}

Method 3: Bind - The bind() method creates a new function that, when called, has its this keyword set to the provided value with a given sequence of arguments preceding any provided when the new function is called.

Reference: Function.prototype.bind()

  function MyConstructor(data, transport) {
    this.data = data;
    transport.on('data',(function() {
        alert(this.data);
    }).bind(this);

Solution 5:

The trouble with "context"

The term "context" is sometimes used to refer to the object referenced by this. Its use is inappropriate, because it doesn't fit either semantically or technically with ECMAScript's this.

"Context" means the circumstances surrounding something that adds meaning, or some preceding and following information that gives extra meaning. The term "context" is used in ECMAScript to refer to execution context, which is all the parameters, scope, and this within the scope of some executing code.

This is shown in ECMA-262 section 10.4.2:

Set the ThisBinding to the same value as the ThisBinding of the calling execution context

Which clearly indicates that this is part of an execution context.

An execution context provides the surrounding information that adds meaning to the code that is being executed. It includes much more information than just the thisBinding.

The value of this isn't "context". It's just one part of an execution context. It's essentially a local variable that can be set by the call to any object and in strict mode, to any value at all.