typeof null is "object"

typeof null returns "object" because of a bug in the first version of JavaScript that can never be fixed without breaking the web. Check for null with === null.

JavaScriptNull and undefined errors

What it means

In the original implementation, values carried a type tag and null used the same tag as objects. The behaviour was kept for compatibility. So a check like typeof x === "object" lets null through, and then x.property throws.

Common causes

1. Checking for an object with typeof

null passes the check.

Breaks

if (typeof value === "object") {
  return value.id;   // crashes on null
}

Works

if (value !== null && typeof value === "object") {
  return value.id;
}

2. Describing a value's type

null needs its own case before typeof.

Breaks

return typeof value;

Works

if (value === null) return "null";
if (Array.isArray(value)) return "array";
return typeof value;

How to find it in your own code

Test null explicitly with value === null (or value == null to catch undefined too), and arrays with Array.isArray, before relying on typeof.

Try it: fix this bug

hard

describeType(null) returns "object" instead of "null".

function describeType(value) {
    if (typeof value === "object") {
        return "object";
    }
    return typeof value;
}

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Other common errors

TypeError: Cannot read properties of undefined TypeError: Cannot read properties of undefined (reading 'map')TypeError: Cannot read properties of undefined (reading 'length')return on its own line returns undefined AttributeError: 'NoneType' object has no attribute TypeError: unsupported operand type(s) for +: 'int' and 'NoneType'KeyError: 'key' in a Python dictionary TypeError: 'NoneType' object is not subscriptable