JavaScript Primitives and References

Written by Swapnil RajaPublished 24 Aug 2026

Primitives are copied by value and objects are copied by reference.

This single difference explains a whole category of confusing bugs.

It is why changing one variable can change another.

Example

javascript

let a = 1;
let b = a;
b = 2;

console.log(a);

The output is 1: b got its own copy.

The Two Groups

Primitives: string, number, boolean, null, undefined, symbol, bigint.

Everything else is an object, including arrays and functions.

Assigning a primitive copies the value; assigning an object copies the reference.

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Complete Example

html

<!DOCTYPE html>
<html>
<head>
  <title>JavaScript Primitives and References</title>
</head>
<body>

  <h1>Primitives and References</h1>

  <p id="out"></p>

  <script>
    let numberA = 1;
    let numberB = numberA;
    numberB = 2;

    const objectA = { count: 1 };
    const objectB = objectA;
    objectB.count = 2;

    const objectC = { count: 1 };
    const objectD = { ...objectC };
    objectD.count = 2;

    document.getElementById("out").innerHTML =
      "Primitive original: " + numberA +
      "<br>Shared object original: " + objectA.count +
      "<br>Spread copy original: " + objectC.count;
  </script>

</body>
</html>

JavaScript Primitives and References

Example

The Two Groups

Syntax

Objects Are Shared

Example

Making a Real Copy

Example

Arrays Behave the Same Way

Example

Passing to a Function

Example

Comparison Follows the Same Rule

Example

Complete Example

Try It Yourself

Important Points

Conclusion