argumentPassing.md

[JavaScript] - How Passing Arguments Works: Value vs Reference

SOURCE, SOURCE

JavaScript does not have pass by reference. The object passing might looks like passing by reference but it is still passing by value. The value here is the memory address in the stack that points to the object in the heap.

Again, JavaScript is a pure pass by value language. It's just that the values being passed around are references when not dealing with primitives

const flight = 'LH234';
const jonas = {
  name: 'Jonas Schmedtmann',
  passport: 24739479284,
};

const checkIn = function (flightNum, passenger) {
  flightNum = 'LH999';
  passenger.name = 'Mr. ' + passenger.name;

  if (passenger.passport === 24739479284) {
    alert('Checked in');
  } else {
    alert('Wrong passport!');
  }
};

checkIn(flight, jonas);
// flight is a string (primitive), so when passing it to a function it will create a new value (flightNum).
// Updating this value doesn't update the original value (pass by value).
console.log(flight);

// jonas is an object, so when passing it to a function, the reference in the stack is shared while the actual
// object values in the heap remains the same. Thus jonas is unexpectedly mutated too.
console.log(jonas);

const newPassport = function (person) {
  person.passport = Math.trunc(Math.random() * 100000000000);
};

newPassport(jonas); // jonas has been mutated
checkIn(flight, jonas); // thus it will show Wrong passport now