Javascript This

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Michael Zippo 04.11.2021

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Understanding This JavaScript Keyword

If you want to become a programmer, no matter what specific field you choose, you will definitely have to deal with JavaScript. It has evolved from humble beginnings into a true powerhouse. Now it covers frontends, backends, mobile devices, games, desktop applications, and even machine learning !

Now, before you become a JavaScript master and develop, you need to learn the basics . The keyword this should probably be number one on your learning list.

What is doing?

Simply put, this is a pointer, a way to refer to what object called a function or method in JavaScript code. Just put, this is what is to the left of the dot

This has two main uses. Let them explore with a few examples.

Building objects with this

The above code produces the following output:

here, we have defined a constructor where we are subject of books that take a title, author, and genre as parameters.

a constructor function is a "project" function object from which we can create objects with the given name (book in our case).

We have created an object called myBook and successfully passed the required parameters (called arguments when we call or invoke a function, as opposed to parameters when we define).

Finally, we saved myBook and retrieve the object with its corresponding properties. In this way, each new book object, we create gets transmitted values that are his assigned

Access to properties with this .

Here we have defined a child variable called which owns an object with name and property age, and a function to introduce.

The function records an introductory sentence using the properties of the object, accessed with ce .wanted_property.

This and JavaScript scope

By default, when used in global scope, that is, not in a defined function or object , this will refer to the global object. In the browser, the window is the global object.

When you declare primitive data (strings, numbers, booleans ...), an object or a function in JavaScript, they are all attached to the global object (window).

Anything you write "in the open‚", you can read it as if there was a window. written before. Let’s take a look at this code snippet:

First , we define a function called returnThis in global scope that returns the value of this. We therefore call by writing returnThis ().

Since it has global scope, we can see it as window.returnThis (), which return what’s left of the point.

object functions aka methods

If we now define a new object that has the same function in its interior (called method object) and call it once more, we are going to get this object (indicated by the braces) returned.

the Gotcha of this and its three solutions

If a function is not directly related to an object, the this in the function will refer to the global object.

Eseguiamo questo nel browser e vediamo cosa otteniamo:

definito a oggetto che con una funzione restituisce questo di nuovo e un’altra che restituisce una funzione interna, che quindi restituisce questo.

Quando vogliamo chiamare la funzione indirettaNestedFunction, prima scriviamo () una volta per chiamare the funzione esterna che restituisce the interno.

Poi chiamiamo the interno con un altro () che restituisce la window.

This is one of the mysterious behaviors of JavaScript and gives rise to some unexpected and unwanted surprises.