JavaScript Typeof

A string that names the kind

typeof value is an operator, not a function. It returns a string you can compare. Use it when you need to know whether a name holds a number, a string, a function, or something else before you call a method on it.

Parentheses are optional: typeof x and typeof(x) do the same job. The result is always a string, even when the value is undefined.

The usual answers

console.log(typeof 7);
console.log(typeof "7");
console.log(typeof true);
console.log(typeof undefined);
console.log(typeof console.log);

Arrays say object

An array is an object with a length and indexed slots. typeof [] is"object", not "array". Ask Array.isArray(value) when the difference matters.

const list = [1, 2, 3];
const record = { n: 3 };

console.log(typeof list);
console.log(typeof record);
console.log(Array.isArray(list));
console.log(Array.isArray(record));

null says object

typeof null is "object". That is an old language bug that was never fixed, because fixing it would break existing pages. Treat null as its own check:value === null.

const empty = null;
const box = {};

console.log(typeof empty);
console.log(typeof box);
console.log(empty === null);
console.log(box === null);

If you only test typeof x === "object", you will treat null as a usable object and then crash on x.name.

A safe object check

When you need “a real object I can read fields from,” combine the tests. Arrays may or may not count, depending on the job.

function describe(value) {
  if (value === null) return "null";
  if (Array.isArray(value)) return "array";
  return typeof value;
}

console.log(describe(null));
console.log(describe([1, 2]));
console.log(describe({ a: 1 }));
console.log(describe("hi"));

typeof and missing names

typeof undeclaredName is "undefined" and does not throw. Reading the name without typeof throws ReferenceError. That is one reason people wrap a check in typeof before using a global that might not exist.

For a variable you declared, typeof still returns "undefined" if you have not assigned yet. That is normal, not an error.

What to remember

• typeof returns a string such as "number" or "string".
• Arrays and null both report "object".
• Use Array.isArray and === null for those two cases.
• Functions report "function", which is the useful exception.

Next: type conversion — turning a string into a number on purpose, and how + does it by accident.