The typeof operator

Learn how to check a variable's type with typeof and its pitfalls.

What is the typeof operator?

To find out a value's type you use the typeof operator. typeof returns the type of the value on its right as a string: "number", "string", "boolean" and so on.

For example, typeof 42 gives "number"; typeof "hello" gives "string"; typeof true gives "boolean". Since the result is a string, writing console.log(typeof 42) shows the word number on screen.

The typeof null pitfall

typeof null does not return "null" as you might expect; it returns "object". This is a historical bug from the early years of JavaScript; it could not be fixed later because old code would break.

So to check whether a value is null, do not use typeof. Use the direct comparison deger === null instead. This behavior is known as a pitfall; it surprises programmers who do not know it.

Exercise Print the type of three values with typeof

Find the type of a number, a string and null using typeof, and print each one with console.log.

Show example solution

console.log(typeof 42);
console.log(typeof "hello");
console.log(typeof null);

Test yourself

What is the result of typeof 42?
```
typeof 42
```
- "number"
- "string"
- "integer"
- "42"
Show answer

"number" — Correct. 42 is a number, so typeof returns "number".
Find the type of 42 with typeof
```
let sonuc = ____;
```
Show answer
```
let sonuc = typeof 42;
```
Match each value to its typeof result

42

"number"
"hello"

"string"
true

"boolean"
null

"object"
What is the result of typeof null?
```
typeof null
```
- "null"
- "undefined"
- "object"
- "boolean"
Show answer

"object" — Correct. typeof null returns "object"; this is a historical bug.

Next lesson: Summary - Variables and types

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