JavaScript null and undefined

Written by Swapnil RajaPublished 24 Aug 2026

undefined means a value was never set; null means it was set to nothing on purpose.

Both represent an absence, which is why they are so often confused.

The difference is who caused it: JavaScript, or you.

Example

javascript

let notSet;
const emptied = null;

console.log(notSet);
console.log(emptied);

The output is undefined then null.

Which One Appears When

undefined is what JavaScript gives you: an unassigned variable, a missing property, a function with no return.

null is what you assign yourself to say deliberately empty.

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Example

javascript

function describe(value) {
  if (value == null) {
    return "nothing there";
  }
  return "got " + value;
}

console.log(describe(null));
console.log(describe(undefined));
console.log(describe(0));

Complete Example

html

<!DOCTYPE html>
<html>
<head>
  <title>JavaScript null and undefined</title>
</head>
<body>

  <h1>null and undefined</h1>

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

  <script>
    let notSet;
    const emptied = null;
    const object = {};

    const lines = [
      "unassigned variable: " + notSet,
      "missing property: " + object.missing,
      "deliberately empty: " + emptied,
      "null == undefined: " + (null == undefined),
      "null === undefined: " + (null === undefined)
    ];

    document.getElementById("out").innerHTML = lines.join("<br>");
  </script>

</body>
</html>

JavaScript null and undefined

Example

Which One Appears When

Syntax

Where undefined Comes From

Example

Using null Deliberately

Example

Comparing Them

Example

Checking for Either

Example

In JSON

Example

Complete Example

Try It Yourself

Important Points

Conclusion