var self this

In the dynamic and often nuanced world of JavaScript, mastering the intricacies of the this keyword is a pivotal step for any developer. It’s a concept that frequently trips up even experienced programmers due to its context-dependent nature. The question of “var self = this?” often arises when developers encounter situations where the value of this isn’t what they expect, particularly within callback functions or nested scopes. This classic JavaScript pattern, while seemingly straightforward, addresses a fundamental challenge in maintaining object context and ensuring your code behaves predictably. Understanding when and why to employ this strategy, alongside modern alternatives, is essential for writing robust, maintainable, and bug-free JavaScript applications. Let’s dive deep into why this can be so tricky and how this pattern, among others, helps us regain control over execution context.

Understanding the Elusive this Keyword in JavaScript

The this keyword in JavaScript is not like its counterparts in other languages, where it typically refers to the instance of the class the method belongs to. Instead, JavaScript’s this is highly dynamic, with its value determined by how a function is called, rather than where it’s defined. This behavior, often referred to as dynamic scoping for this, is a core aspect of JavaScript’s execution context. For instance, when a function is called as a method of an object (e.g., myObject.myMethod()), this inside that method will refer to myObject. However, if the same function is called independently (e.g., myMethod() in the global scope or a non-strict mode), this might refer to the global object (window in browsers, or undefined in strict mode).

This variability can lead to significant confusion and bugs, especially when dealing with asynchronous operations, event handlers, or nested functions. Developers must constantly be aware of the implicit context in which their code is running. The global execution context, function execution context, and eval execution context each define this differently. For example, in a simple function call, this often defaults to the global object unless in strict mode. When a function is invoked via the new keyword to create an object, this within the constructor refers to the newly created instance. Mastering these distinctions is foundational to writing effective JavaScript that correctly interacts with its surrounding environment and data. Without a clear grasp, developers might find their methods attempting to operate on the wrong data, leading to errors that are difficult to diagnose.

According to Mozilla Developer Network (MDN), “The this keyword behaves a little differently in JavaScript compared to other languages. It also has some differences between strict mode and non-strict mode.” This highlights the complexity and the need for a deep understanding of its various binding rules. Understanding this mutable nature is the first step towards effectively managing the execution context within your applications.

The Problem: When this Loses Its Context

The dynamic nature of this becomes particularly problematic in scenarios involving callback functions, which are prevalent in modern JavaScript development. Consider a common pattern: an object with a method that needs to execute a task after a delay using setTimeout or responds to a user interaction via an event listener. Inside the callback function passed to setTimeout or an event handler, the this keyword often no longer refers to the original object. Instead, it might default to the global object (window) in non-strict mode, or be undefined in strict mode, or even refer to the DOM element that triggered the event, depending on the specific event handler’s invocation.

For example, if you have an object user with a method greet that attempts to access this.name, and you pass user.greet as a callback to setTimeout, when greet is eventually executed, this inside it won’t be user. Instead, it will be the global object, and this.name will be undefined or refer to a global variable if one exists. This loss of the intended this context is a frequent source of bugs, especially in larger applications where functions are passed around as first-class citizens. The scope chain alone isn’t enough to preserve this because this is determined at call-time, not definition-time.

This issue is not limited to setTimeout. It’s equally relevant for array iteration methods like forEach, promise callbacks (.then()), or any situation where a function is detached from its original object and called in a different context. The challenge lies in ensuring that the callback function, when finally executed, still has access to the correct object’s properties and methods. This is precisely where patterns like capturing this come into play, offering mechanisms to explicitly manage or preserve the desired execution context across different function invocations. Without such strategies, developers would constantly be battling against the default, often undesirable, behavior of this.

Solutions: How var self = this; Helps and Modern Approaches

When faced with the challenge of a shifting this context, developers have traditionally employed several strategies to ensure their functions operate on the correct object. One of the most common and classic patterns is to capture the value of this in a separate variable, often named self, that, or _this, before entering the problematic scope. This pattern, var self = this;, works by leveraging JavaScript’s closure mechanism. Since self is declared in the outer scope, any inner function (the callback) forms a closure over that scope, allowing it to access the value of self even after the outer function has finished executing. This effectively “fixes” the context for the inner function, allowing it to reference the intended object’s properties and methods.

For instance, in an object method, you might write: var self = this; setTimeout(function() { console.log(self.data); }, 1000); Here, self.data correctly accesses the data of the original object because self retains the value of this from the outer scope. While effective, this pattern has become less common with the advent of ES6 features. Modern JavaScript offers more elegant and explicit ways to manage this. The Function.prototype.bind() method allows you to create a new function that, when called, has its this keyword set to a specific value. Similarly, call() and apply() methods allow you to invoke a function immediately with a specified this value and arguments.

To preserve the context of this using the var self = this; pattern, follow these steps:

1. Inside the method or function where this holds the desired context, declare a new variable (e.g., self).
2. Assign the current value of this to this new variable: var self = this;.
3. Within any nested function or callback where this would normally change, use self instead of this to refer to the original object.

However, the most significant modern alternative is the arrow function (=>). Arrow functions do not have their own this binding. Instead Question & Answer :

Using instance methods as callbacks for event handlers changes the scope of this from “My instance” to “Whatever just called the callback”. So my code looks like this

function MyObject() { this.doSomething = function() { ... } var self = this $('#foobar').bind('click', function(){ self.doSomethng() // this.doSomething() would not work here }) } 

It works, but is that the best way to do it? It looks strange to me.

This question is not specific to jQuery, but specific to JavaScript in general. The core problem is how to “channel” a variable in embedded functions. This is the example:

var abc = 1; // we want to use this variable in embedded functions function xyz(){ console.log(abc); // it is available here! function qwe(){ console.log(abc); // it is available here too! } ... }; 

This technique relies on using a closure. But it doesn’t work with this because this is a pseudo variable that may change from scope to scope dynamically:

// we want to use "this" variable in embedded functions function xyz(){ // "this" is different here! console.log(this); // not what we wanted! function qwe(){ // "this" is different here too! console.log(this); // not what we wanted! } ... }; 

What can we do? Assign it to some variable and use it through the alias:

var abc = this; // we want to use this variable in embedded functions function xyz(){ // "this" is different here! --- but we don't care! console.log(abc); // now it is the right object! function qwe(){ // "this" is different here too! --- but we don't care! console.log(abc); // it is the right object here too! } ... }; 

this is not unique in this respect: arguments is the other pseudo variable that should be treated the same way — by aliasing.