How to create constructor in javascript. The constructor approach in JavaScript is a proper method used to create and initialize objects inside a class. That is what a constructor in each programming language takes. What does JavaScript constructor different from others is the rest of the syntax? To understand it properly, just open the browser developer tools by F12 key or (Ctrl/Cmd + Shift + C) and go to the Console tab in the developer tools window.
Types of Constructors in JavaScript:)
There are two types of constructors in JavaScript
- User-defined Constructors
- Built-in Constructors
Create constructor in javascript. Before we glance at the syntax of JavaScript constructors, we should understand a pretty basic concept of JavaScript.
- The Javascript Object class abstracts in the context of this article, each JavaScript object is performed by the Object constructor. If the value provided while constructing the object is null or undefined, the Object constructor will generate an empty object. Unless it will create an object of the type specified while making the object.
- Whenever a new object of the class type is indicated, the new keyword delivers a source to the newly created object. That object is obtained using this keyword inside the constructor to initialize the properties of the object.
- Although, technically JavaScript does not have any classes but has constructors and prototypes to take alike functionality. In ECMAScript 2015, the idea of classes was added in JavaScript. This was really a syntactical supplement to being prototype-based inheritance and didn’t add any new functionality to the language.
Custom Constructors (User-defined)
We are also able to declare and determine our own constructors to be used during our application. Let’s look at how this can be done.
Syntax:
function myfunction([arguments]){ ... }Example Code:)
<script type="text/javascript">
function myFunction(name, author, edition ) {
this.name = name;
this.author = author;
this.edition = edition ;
}
function displaymyFunction(myFunction){
console.log('\'' + myFunction.name + '\' authored by ' + myFunction.author + ' in the edition ' + myFunction.edition + '.');
}
var x = new myFunction('A Modern Introduction to Programming', 'Marjin Haverbeke', '3rd edition ');
var y = new myFunction('JavaScript: The Good Parts', 'Douglas Crockford', '1st edition' );
var z = new myFunction('Learn JavaScript VISUALLY', 'Ivelin Demirov', '1st edition' );
displaymyFunction(x);
displaymyFunction(y);
displaymyFunction(z);
</script>Built-in Constructors
The javascript constructor is called immediately when an object of the class Object is formed. This produces an object of class Object if invalid or undefined parameters are declared as arguments. Otherwise, an object of the class of provided parameters is created.
Syntax:
new Object([value])
Example Code:)
<script type="text/javascript">
var book = new Object("Learn JavaScript VISUALLY");
var edition = new Object('1st edition');
document.write("Book Name = "+book+"<br>"+ " Book edition = "+edition);
</script>Importance of the new Keyword:)
Now you may be staring at what if I do not use the new keyword? Can I delete the new keyword? Well, dear friend, no. Using the new keyword is very much imperative.
- JavaScript constructors are proper functions. They are followed by a new keyword to determine the JavaScript engine that a new object must be created with the supplied properties. Without the new keyword, you would be simply producing more and more global objects.
- The new keyword yields a citation to the newly created object. We then save this reference in a variable. Without the new keyword, the object is formed, but no reference to the object is delivered. The object allows a global scope. The only reference to this object is through the window object.
- Doubtful? Let us explain better with examples. Consider the above example again. We killed the new keyword from the object expressions. The result was an omission of undefined variables. This is because, without the new keyword the newly formed objects’ reference was not delivered and thus was not saved in our variables X, Y, and Z. When we decided to access these variables in the display book method, the exception is thrown.
Here is the same example we saw above. But if you remove new keywords from the code then the function will return the value.
<script type="text/javascript">
function myFunction(name, author, edition ) {
this.name = name;
this.author = author;
this.edition = edition ;
}
function displaymyFunction(myFunction){
console.log('\'' + myFunction.name + '\' authored by ' + myFunction.author + ' in the edition ' + myFunction.edition + '.');
}
var x = new myFunction('A Modern Introduction to Programming', 'Marjin Haverbeke', '3rd edition ');
var y = new myFunction('JavaScript: The Good Parts', 'Douglas Crockford', '1st edition' );
var z = new myFunction('Learn JavaScript VISUALLY', 'Ivelin Demirov', '1st edition' );
displaymyFunction(x);
displaymyFunction(y);
displaymyFunction(z);
</script>Scope-Safe Constructors:)
The built-in constructors in JavaScript are scope-safe constructors. They don’t produce globally scoped variables when called without a new keyword. Thus, these objects can be safely constructed with or without a new keyword.
<script type="text/javascript">
function myFunction(argument) {
// if "this" is not an instance of the constructor
// it means it was called without new
if (!(this instanceof myFunction)) {
// call the constructor again with new
return new myFunction(argument);
}
// The code to construct properties and methods
}
</script>Yes, you also can create user-defined scope-safe constructors. Go on, create a scope-safe constructor for our 1 in the above example.
Conclusion
This article supplied an in-depth explanation of JavaScript Constructors. This also assists in getting the working of JavaScript. The key thing to identify here is that although technically there are no classes in JavaScript, the methods and prototypes give comparable functionality under the developer’s control. And yes, the new keyword is essential. Create constructor in javascript.