method in class constructor vs prototype

• Define the method in the class constructor

function Calculator() {
    this.sum = function (a, b) {
        return a + b;
    };
}

• Or define the method in the class prototype:

function Calculator() {}

Calculator.prototype.sum = function (a, b) {
    return a + b;
};

const calc = new Calculator();
calc.sum(1, 2); // 3

Differences

1. In the first approach, the method has to be created every time we create a new instance of the class. The second approach, on the other hand, creates the method once and share it for all instances of the class.
   That's being said, the second approach is more efficient, and requires less memory than the first one.
2. The method created via prototype can be extended and overridden by the sub class.

Good practice

function Foo() {
    this.bigFunction = function () {
        // A big function here
    };

    this.anotherBigFunction = function () {
        // ...
    };
}

function Foo() {}

Foo.prototype.bigFunction = function() {
    ...
}

Foo.prototype.anotherBigFunction = function() {
    ...
}

See also

• function vs property in interface