Object.prototype.valueOf()

function MyNumberType(n) {
  this.number = n;
}

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

const object1 = new MyNumberType(4);

console.log(object1 + 3);
// Expected output: 7

object.valueOf();

MyNumberType.prototype.valueOf = function () {
  return customPrimitiveValue;
};

myNumberType.valueOf();

function MyNumberType(n) {
  this.number = n;
}

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

var myObj = new MyNumberType(4);
myObj + 3; // 7

+"5"; // 5 (string to number)
+""; // 0 (string to number)
+"1 + 2"; // NaN (doesn't evaluate)
+new Date(); // same as (new Date()).getTime()
+"foo"; // NaN (string to number)
+{}; // NaN
+[]; // 0 (toString() returns an empty string list)
+[1]; // 1
+[1, 2]; // NaN
+new Set([1]); // NaN
+BigInt(1); // Uncaught TypeError: Cannot convert a BigInt value to a number
+undefined; // NaN
+null; // 0
+true; // 1
+false; // 0