The typeof operator
- JavaScript course
- Beginner
- Lesson 5/133
- 4 min
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.
Practice 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.
Starter code
// print the type of a number
// print the type of a string
// print the type of nullSample solution
console.log(typeof 42);
console.log(typeof "hello");
console.log(typeof null);Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What is the result of typeof 42?
typeof 42- "integer"
- "string"
- "number" Correct answer
- "42"
Explanation: 42 is a number, so typeof returns "number".
2 Find the type of 42 with typeof
let sonuc = ____;let sonuc = typeof 42;Explanation: The typeof operator takes the value with a space.
3 Match each value to its typeof result
| Term | Meaning |
|---|---|
42 |
"number" |
"hello" |
"string" |
true |
"boolean" |
null |
"object" |
4 What is the result of typeof null?
typeof null- "null"
- "undefined"
- "boolean"
- "object" Correct answer
Explanation: typeof null returns "object"; this is a historical bug.