When this Argues: Understanding 'this' in Callback Functions

In JavaScript, the value of this in a function call is determined by the context in which the function is executed. When passing this as an argument, however, the rules can get complicated.

Specifically, the following scenario arises: when a callback function is passed as an argument, why doesn't this get set to the function that calls the callback?

Understanding the Hierarchy of 'this'

To understand why this is set where it is, we need to consider the hierarchy of function calls:

• obj.prepareRandomFunction() sets this to obj (rule #2 from the answer).
• randomFunction(this.sumData.bind(this)) passes this.sumData.bind(this) as an argument (rule #1).
• Inside randomFunction, callback(data) sets this to the global object (rule #1).

However, before randomFunction calls the callback, it uses this.sumData.bind(this) to create a new function (rule #5). This new function calls the original callback function, but now with this bound to obj (the argument passed to bind).

Implications for Callback Functions

When passing a method as a callback, it's crucial to understand that it won't be called as obj.method(). This means this will not have the correct value inside the callback function. To work around this issue, you can use bind() to set the value of this within the callback.

Other Useful Notes

• Rule #6 in the answer describes how ES6 arrow functions maintain the current lexical value of this, even in callback functions.
• .apply() and .call() can be used to create new function calls with specific values of this.
• bind() can be used to create new functions that call the original function with a custom value of this.
• Understanding the complex nature of this is essential for effective JavaScript coding and for mastering concepts like callbacks.