Type checking using typeof in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the typeof operator do in JavaScript?

It tells you the type of a value or variable, like whether it is a number, string, boolean, object, function, or undefined.

beginner

What is the result of typeof 42?

The result is "number" because 42 is a number.

intermediate

What does typeof null return and why is it special?

It returns "object". This is a known quirk in JavaScript where null is considered an object type even though it means 'no value'.

beginner

How can you check if a variable is a function using typeof?

Use typeof variable === 'function'. This returns true if the variable holds a function.

intermediate

What type does typeof return for arrays?

It returns "object" because arrays are a special kind of object in JavaScript.

What will typeof 'hello' return?

A "string"

B "text"

C "char"

D "object"

typeof returns "string" for text values.

Which typeof result means the variable is not defined or has no value?

A "null"

B "undefined"

C "void"

D "none"

"undefined" means a variable has no assigned value.

What does typeof function() {} return?

A "method"

B "object"

C "function"

D "callable"

Functions return "function" when checked with typeof.

What is the typeof result for an array like []?

A "object"

B "collection"

C "list"

D "array"

Arrays are objects in JavaScript, so typeof returns "object".

Why is typeof null considered a quirk?

A Because it returns "undefined"

B Because it throws an error

C Because it returns "null"

D Because it returns "object"

typeof null returns "object" even though null means no value.

Explain how the typeof operator helps in checking variable types in JavaScript.

Think about how you can tell what kind of value a variable holds.

Describe the special case of typeof null and why it might confuse beginners.

Consider what null means and what typeof returns.