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JavaScript typeof
Select one option
1 / 10
console.log(typeof typeof 10)
TL;DR answer sheet
JavaScript typeof — quiz answer sheet, click to expand.
1. console.log(typeof typeof 10)
Answer: string
Explanation: `typeof 10` returns "number" as a string, so you're actually doing `typeof "number"`, which is "string".
2. console.log(typeof /douiri/)
Answer: object
Explanation: Regular expressions in JavaScript are objects.
3. console.log(typeof 10n)
Answer: bigint
Explanation: The `n` suffix denotes a BigInt, which are numbers too high or too low to be represented by the number primitive.
4. console.log(typeof undeclaredVariable)
Answer: undefined
Explanation: Accessing an undeclared variable with `typeof` doesn't throw an error — it returns "undefined".
5. console.log(typeof [])
Answer: object
Explanation: Arrays are objects in JavaScript. To check if something is an array, use `Array.isArray()`.
6. console.log(typeof class C {})
Answer: function
Explanation: Classes in JavaScript are just syntactic sugar for constructor functions under the hood.
7. console.log(typeof null)
Answer: object
Explanation: This is a well-known JavaScript bug. `typeof null` returns "object" even though null isn't an object.
8. console.log(typeof NaN)
Answer: number
Explanation: `NaN` stands for Not-a-Number, but ironically, `typeof NaN` is "number".
9. console.log(typeof new Date())
Answer: object
Explanation: Dates are objects too, to check if something is a Date, use `instanceof Date`.
10. console.log(typeof Symbol("id"))
Answer: symbol
Explanation: Symbols are a unique primitive type in JavaScript.
Key Takeaways of this quiz
- The `typeof` operator returns a string indicating the type of the operand.
- `typeof null` returns "object", a well known javascript bug.
- `typeof` on undeclared variables returns "undefined" — it doesn't throw an error.
- `typeof []` returns "object", not "array" — use `Array.isArray()` to check for arrays.
- `typeof class Foo {}` returns "function" — classes are just special functions.
- BigInt and Symbol are modern primitive types: `typeof 10n` is "bigint" and `typeof Symbol()` is "symbol".
- `typeof typeof x` always returns "string" because the inner `typeof` returns a string.
- Regular expressions like `/abc/` are objects.
- `typeof NaN` is "number", even though it's 'Not a Number'.
- `typeof new Date()` is "object" — dates are objects like arrays and regex.
