Type narrowing with typeof in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does typeof do in TypeScript?

typeof checks the type of a variable at runtime and helps TypeScript narrow down the variable's type for safer code.

beginner

How does TypeScript use typeof for type narrowing?

TypeScript uses typeof in conditional checks to understand the exact type of a variable inside that block, allowing safer operations.

beginner

Example: What type does TypeScript narrow x to inside this block?<br>

if (typeof x === 'string') { /* here */ }

Inside the if block, TypeScript knows x is a string.

intermediate

Can typeof narrow types other than string?

Yes, typeof can narrow types like number, boolean, undefined, object, and function.

beginner

Why is type narrowing useful in TypeScript?

It helps avoid errors by letting TypeScript know the exact type, so you can safely use properties or methods specific to that type.

What does typeof x === 'number' check?

A If x is undefined

B If x is a string

C If x is a number

D If x is an object

This checks if the variable x is of type number.

Inside if (typeof y === 'boolean'), what type does TypeScript treat y as?

A string

B any

C number

D boolean

TypeScript narrows y to boolean inside the block.

Which of these is NOT a valid typeof result in JavaScript/TypeScript?

A 'array'

B 'number'

C 'string'

D 'boolean'

typeof never returns 'array'; arrays are objects.

What happens if you try to access a string method on a variable not narrowed to string?

A TypeScript gives an error

B TypeScript allows it without error

C JavaScript crashes

D The code runs slower

TypeScript will give an error because it cannot be sure the method exists on that type.

Which keyword helps TypeScript narrow types at runtime?

A let

B typeof

C void

D instanceof

typeof checks the type of a variable and helps narrow it.

Explain how typeof helps with type narrowing in TypeScript.

Think about how checking a variable's type lets TypeScript know what it can safely do.

List the common types that typeof can check in TypeScript.

Remember the basic JavaScript types that typeof returns.