Value vs Reference in JavaScript

Use this guide to explain how primitives and objects behave in JavaScript.

Value types (primitives)

• string, number, boolean, bigint, symbol, null, undefined.
• Copied by value.

Reference types

• Objects, arrays, functions.
• Assigned by reference; mutations affect all references.

const original = { count: 1 };
const alias = original;
alias.count = 2;
// original.count === 2

When it matters

• React state updates rely on immutable copies.
• Avoid unintended mutation in shared references.

Interview prompt

• Explain how to update nested state without mutation.