function call with new keyword

 

function person(fistName, lastName) {

    this.fistName = fistName;

    this.lastName = lastName;

}

var person1 = new person("John", "Doe");

var person2 = person("Jane", "Smith");

console.log(person1); // John Doe

console.log(person2); // undefined

Output:

person { fistName: 'John', lastName: 'Doe' }

undefined

Notes:

    whenever you call a function with the `new` keyword, it creates a new object and sets the

    context 0f `this` to that new object. In the case of `person1`, it correctly creates a

    new instance of the `person` function, and you can see the properties `fistName` and

    `lastName` are set to "John" and "Doe" respectively.

    but when you call `person` without the `new` keyword, as in the case of `person2`,

    it does not create a new object. Instead, it calls the function in the

    global context (or undefined in strict mode), and since there is no return

    statement in the function, it returns `undefined`. Therefore, `person2` is `undefined`,

    and trying to access its properties will result in an error.