Are values passed by reference or by value in JavaScript?

Jun 27, 2020

Are values passed by reference or by value in JavaScript? Primitive types pass by value while objects pass by reference, as these examples show.

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The answer is: it depends!

Primitive types are passed by value.

Objects are passed by reference.

Primitive types are numbers, strings, booleans, null, undefined and symbols.

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

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, it’s passed by reference, so if you modify one of its properties, also the original object is modified:

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

const num = {
  value: 2
}

increment(num)

console.log(num.value) //3

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