What happens when a constructor function is invoked without the `new` operator?

I am a newbie in javascript, and was studying about constructor functions and the new keyword. After omitting new intentionally, it is causing some problems, and I am not able to understand exactly why is it happening.

let Fruit = function (person, color) {
    this.person = person;
    this.color = color;
    this.displayInfo = function () {
        console.log(this.person + " is " + this.color);
    }
}
let altFruit = function (person, color) {
    this.person = person;
    this.color = color;
}

let a1 = altFruit('Banana', 'Yellow');   //new keyword not written
let f1 = new Fruit('Orange', 'orange');

let aFunction = f1.displayInfo;
aFunction();

I didn't write new when initialising object a1. I stored the function f1.displayinfo in a variable, and then called it. Expected output was undefined is undefined, but instead I got Banana is Yellow.

It is really baffling to me because aFunction has nothing to do with altFruit constructor function or a1. Also, even after seeing many tutorials, I still don't really understand what is the exact meaning of new because this weird output is happening only when I omit it.

Solution

Value of this depends on how the function is called.

In non-strict mode, when you call the altFruit function without the new keyword, this inside it will refer to the global window object. So,

this.person = person;

will add person property on the global window object. (you can test this by logging window.person on the console at the end of your code).

When you call aFunction, in non-strict mode, value of this inside displayInfo function will refer to the global window object.

In short, your code is adding and reading properties to and from the global window object, respectively.

The "new" operator

new operator is used to create instances of user-defined object types (like Fruit) or built-in object types (like Array, Date, etc).

When you call new Fruit(...), new operator will do the following things:

Create a new object
Add __proto__ property on the newly created object that points to Fruit.prototype object.
Point this inside the Fruit constructor to the newly created object.
Return the newly created object from the Fruit constructor function.

For more details, see: MDN: new operator