“this” and function invocation patterns JS (Part II)

In the previous article, we talked about method invocation pattern and function invocation pattern. This time will review a couple of more invocation patterns that we used to use in Javascript.

Will also review new features added in ES5 and we talk about some cases derived from the ES6: arrow functions.

Let’s dig into it.

3# The constructor invocation pattern

Before ES6 arrived, Javascript had no classes (see classes in ES6) but it was possible to mimic it using constructors to create objects which inherit method and properties from the “parent”.

As seen above the method getName() prints the correct value Batman, so the this value is bound to the batman object. Why? Because when invoking a function with new, a new object is created with a hidden link to the value of the functions prototype member and this is bound to that new object and not to the global object (window in the browser) as you could see in the previous example.

4# The apply invocation pattern

There are a couple of methods in Javascript that allow us to change the value that should be bound to this when we invoke a function. Those methods are call and apply. The only difference between these two methods is how the params are passed to the function. The apply method accepts params as array and call as argument list.

Let’s use this new superpower to fix the problem with this in the previous example from the function invocation pattern.

ECMAScript 5 introduced the new bind method to change the value of this when a function is invoked. With this method you can pass an object, the difference with the previous call and apply methods is that bind creates a new function with the same body and scope than the passed function but where this occurs in the original function. this is permanently bound to the first argument of bind.

Do you remember the first example? Let’s try this new method:

If you run the previous code you will see that the output for the console.log(baz()) is an empty string. Why? You are assigning an object method to a variable and after that, you are invoking that new variable (baz in the example) as a function so the value of this is bound to the global scope (window object in the browser).

Let’s run again while using whoAmI method:

The result of the execution in the console will tell us what value has this in every execution.

We can fix it using the bind method

Final thoughts

I think that having a good knowledge about the good and bad things of the language you work with (Javascript in my case), will help you to write a better and more maintainable code. And also it will help you to be more efficient in your day to day.

We could talk more about this topic, how this works in arrow functions and if you are able to use call or bind to change the value this is bound to. But this could be the topic of another blog post.

Did you find some other great strategies of your own? Let me know in comments.

And if you enjoyed this post, I’d be very grateful if you’d help me share it.

Thanks for reading.

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