shCode
Reading~10 min

3.6.1 Reading — Pass by Value vs Reference

Primitives (numbers, strings, booleans) are copied when passed to a function — the original is safe. Objects and arrays are shared — a function can change the original.

Pass by value

What you'll learn:

  • What "pass by value" means
  • Which kinds of values JavaScript passes by value
  • Why changing a number inside a function does not change the original

When you pass a primitive value — a number, string, or boolean — JavaScript hands the function a copy. Whatever the function does to that copy, the original variable outside stays the same.

Primitives are: number, string, boolean, undefined, null.

Pass by reference

What you'll learn:

  • What "pass by reference" means
  • Which kinds of values JavaScript passes by reference
  • Why pushing to an array inside a function changes the original

When you pass an object or an array, JavaScript hands the function a reference — a direct pointer to the same spot in memory. If the function modifies the object or array, the original is modified too.

Try it: Read the code and predict what each console.log will print. Then run it and check.

Output
Click Run to see output.

The number myNumber is unchanged because the function only modified a copy. The array myList was modified in place because the function received a reference to the real array.

Short glossary (quick reference)

Term Meaning
primitive A simple value: number, string, boolean, undefined, null
pass by value The function gets a copy — changes inside don't affect the original
pass by reference The function gets a pointer to the original — changes inside do affect it
object / array Non-primitive values; always passed by reference in JavaScript
mutation Changing an object or array in place (e.g., .push(), arr[0] = ...)
Module 3.6Lesson 1 of 3
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