Javascript Object valueOf()

The syntax of the valueOf() method is:

obj.valueOf()

Here, obj is an object.

valueOf() Parameters

The valueOf() method does not take any parameters.

Return value from valueOf()

• Returns the primitive value of the specified object.

Example 1: Custom valueOf()

function customNum(n, fact) {
  this.number = n;
  this.fact = fact;
}

customNum.prototype.valueOf = function () {
  return this.number;
};

var num1 = new customNum(2, "First Prime Number");

console.log(num1 + 3); // 5

Output

5

Example 2: Using built-in valueOf()

// built-in valueOf()
const num = 5;

// string.toString() changes string to number
console.log(+"5" + num); // 10

console.log(+[1] + num); // 6

console.log(+true + num); // 6
console.log(+false + num); // 5

console.log(+undefined + num); // NaN
console.log(+null + num); // 5

Output

10
6
6
5
NaN
5

Recommended Reading: JavaScript Object toString()