What Are Constructor Functions in JavaScript

A constructor function is a special type of function that allows you to programmatically create objects.

1function User(name) {
2  this.name = name;
3  this.role = "Visitor";
4}

Constructor functions are just regular functions, except it is conventional for them to be named with uppercase letters.

And when calling the constructor functions, you must use the keyword new, which tells JavaScript that this is a constructor function, not a regular function.

1let john = new User("John");
2
3console.log(john);

1User { name: 'John', role: 'Visitor' }

Even though we did not write a return statement, a new object will be returned, which is then assigned to the variable john, and you can modify it like a regular object.

1let john = new User("John");
2console.log(john);
3
4john.role = "Admin";
5console.log(john);

1User { name: 'John', role: 'Visitor' }
2User { name: 'John', role: 'Admin' }

Without the keyword new, the function will be executed like a regular function, and undefined will be returned.