Constructor in JavaScript With Example & How to Use Them

What is a constructor in JavaScript?

A function Object() { [native code] } is a function that generates an instance of a class, referred to as a “object” in most cases. When you use the new keyword to declare an object in JavaScript, a function Object() { [native code] } is invoked.

A constructor’s job is to build an object and set values for any object properties that are present. It’s a convenient approach to build an object because you don’t have to specify what to return because the function Object() { [native code] } function returns the object that is generated within it by default.

Example of constructor in JavaScript

Let’s pretend there’s a User object with two properties: firstname and lastname.

You’ll use the same function Object() { [native code] } method to create two instances with different names, as shown in the diagram below:

When a function Object() { [native code] } is called, what happens?

When a function Object() { [native code] } is called in JavaScript, the following happens:

There is a new empty object generated.

This keyword now refers to that newly formed object, making it the current instance object.

The constructor’s returned value is then the freshly constructed object.

Super constructor

The super keyword is used to access the parent’s properties and methods by calling the methods of the parent class. In JavaScript, the super keyword is associated with the concept of inheritance. Assume that the derived class has a member variable or method with the same name as the base class.
It’s mostly utilized when the derived class needs to access a variable, function, or constructor in the base class.

Default constructor

In JavaScript, if you don’t specify any constructor, a default constructor is automatically created which has no parameters:

constructor(){

}

What exactly is this keyword in JavaScript?

This keyword in JavaScript refers to an object.

Which object is used depending on how this is done (used or called).

Depending on how it’s used, this keyword can refer to a variety of objects:

This refers to the object in an object method.

This refers to the global object when used alone.

This refers to the global object in a function.

This is undefined in strict mode in a function.

This is the element that received the event in an event.

This can be referred to by methods like call(), apply(), and bind().

What is the constructor function?

Constructor functions are object-creation templates. It can be used to generate many objects with the same constructor which has the same instance methods and properties but distinct non-method property values. To see if an object was created by a constructor, we can use the instance of an operator.

Example

<script> 
class Employee { 
constructor() { 
this.id=101; 
this.name = "Ram"; 
} 
} 
var emp = new Employee(); 
document.writeln(emp.id+" "+emp.name); 
</script> 

Conclusion

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