Are values passed by reference or by value in JavaScript?

The answer is: it depends!

Primitive types are passed by value.

There are 7 primitives: string, number, bigint, boolean, undefined, symbol, and null. bigint arrived in ES2020.

Everything that’s not a primitive type is an object. Arrays are objects. Functions are objects.

JavaScript always passes by value. For objects the value is a reference to the same object. People call this “pass by sharing”. Mutating a property shows up outside the function. Reassigning the parameter does not change the caller’s variable.

When you pass a number to a function, it’s copied into the function:

const increment = num => {
  num = num + 1
}

const num = 2
increment(num)

console.log(num) //2

If you pass an object, the function gets a copy of the reference. Modify a property, and the original object changes:

const increment = num => {
  num.value = num.value + 1
}

const num = {
  value: 2
}

increment(num)

console.log(num.value) //3

Reassign the parameter, and the caller’s variable stays the same:

const replace = num => {
  num = { value: 10 }
}

const num = {
  value: 2
}

replace(num)

console.log(num.value) //2

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