typeof type guards in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a typeof type guard in TypeScript?

A typeof type guard is a way to check the type of a variable at runtime using the typeof operator. It helps TypeScript narrow down the type inside conditional blocks.

beginner

How does typeof help with type safety in TypeScript?

By checking the type of a variable with typeof, TypeScript can safely allow operations specific to that type inside the guarded block, preventing errors.

beginner

Example: What does this code do?<br>

if (typeof value === 'string') {<br>  console.log(value.toUpperCase());<br>}

This code checks if value is a string. Inside the if block, TypeScript knows value is a string, so calling toUpperCase() is safe.

intermediate

Which types can typeof check in TypeScript?

typeof can check primitive types like 'string', 'number', 'boolean', 'bigint', 'symbol', 'undefined', and 'object' (including null).

intermediate

Why can't typeof distinguish between null and object?

Because in JavaScript, typeof null returns 'object'. So typeof cannot differentiate null from other objects.

What does typeof return for a string variable?

A 'text'

B 'char'

C 'string'

D 'str'

typeof returns the string 'string' for string variables.

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

A 'undefined'

B 'boolean'

C 'number'

D 'array'

typeof never returns 'array'. Arrays are objects, so it returns 'object'.

Why use typeof type guards in TypeScript?

A To check variable types at runtime and narrow types safely

B To declare new types

C To convert types automatically

D To create classes

typeof type guards check types at runtime and help TypeScript narrow types for safer code.

What will typeof null return?

A 'object'

B 'undefined'

C 'null'

D 'boolean'

Due to JavaScript design, typeof null returns 'object'.

Which code snippet correctly uses a typeof type guard?

A if (typeof x === 'stringify') { console.log(x); }

B if (typeof x === 'number') { console.log(x.toFixed(2)); }

C if (typeof x === 'integer') { console.log(x); }

D if (typeof x === 'array') { console.log(x.length); }

Only 'number' is a valid typeof result. 'array', 'integer', and 'stringify' are invalid.

Explain how typeof type guards help TypeScript understand variable types.

Think about how checking a variable's type at runtime helps avoid errors.

List the primitive types that typeof can detect and mention one limitation.

Recall the common JavaScript types and the quirk with null.