Why Use 'var that = this;' in JavaScript?

Understanding the Purpose of "var that = this;" in JavaScript

When working with JavaScript, you may encounter code snippets that include the statement "var that = this;". This construct plays a crucial role in preserving the context of "this" within nested functions, particularly in event handlers.

In JavaScript, the "this" keyword represents the current context or the object that owns the code it's used in. However, when a function is invoked within another function, the value of "this" changes. This can lead to unexpected behavior, especially in event-driven code where event handlers reside in callbacks.

The purpose of assigning "this" to "that" is to capture the current context of the object before it's lost due to a change in scope. By aliasing "this" to "that," you ensure that the original value of "this" remains accessible even within nested functions.

Consider the following example:

$('#element').click(function() {
    // this is a reference to the element clicked on

    var that = this;

    $('.elements').each(function() {
        // this is a reference to the current element in the loop
        // that is still a reference to the element clicked on
    });
});

In this code, the event handler function has access to the element that was clicked on through the "this" keyword. However, within the nested function (each()), the value of "this" refers to the current element in the loop. By capturing "this" as "that" in the outer function, you can still access the element that was initially clicked on.

Using aliases other than "that" is a recommended practice for readability. In the example above, a more descriptive alias like "clickedEl" would make it clearer what the variable refers to.

In conclusion, "var that = this;" is a common technique in JavaScript to preserve the value of "this" across different scopes. It ensures that the original context of the object is maintained even when nested functions are invoked.

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