Typeof operator in type context in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the typeof operator do in a type context in TypeScript?

In a type context, typeof gets the type of a variable or expression, allowing you to use that type elsewhere in your code.

beginner

How is typeof in a type context different from typeof in JavaScript runtime?

typeof in TypeScript type context extracts the type of a variable, while in JavaScript runtime it returns a string describing the type (like 'string' or 'number').

beginner

Given <code>const user = { name: 'Alice', age: 30 };</code>, what does <code>type User = typeof user;</code> do?

It creates a new type User that matches the shape of the user object, with properties name as string and age as number.

intermediate

Can typeof be used to get the type of a function in TypeScript?

Yes, typeof can get the type of a function variable, including its parameter types and return type.

beginner

Why is using typeof in type context helpful in TypeScript?

It helps keep types consistent and reduces duplication by reusing the type of existing variables or objects automatically.

What does typeof return in a TypeScript type context?

A The value of the variable

B A string describing the type

C The type of a variable or expression

D An error

typeof in type context extracts the type, not a string or value.

Given const x = 42;, what is the type of typeof x in TypeScript?

A typeof x

B number

C string

D any

typeof x is the type number because x is a number.

Which of these is a valid use of typeof in a type context?

A type T = typeof someVariable;

B let x = typeof someVariable;

C console.log(typeof someVariable);

D typeof someVariable = string;

You can assign a type alias using typeof in a type context like type T = typeof someVariable;.

Can typeof be used to get the type of a class instance in TypeScript?

A Yes, it gets the instance type

B No, it only works with primitives

C Only with functions

D Only with interfaces

typeof can get the type of any variable, including class instances.

What is the main benefit of using typeof in type context?

A Runs faster code

B Creates new variables

C Changes variable values

D Avoids repeating type definitions

Using typeof helps avoid repeating type definitions by reusing existing types.

Explain how the typeof operator works in a TypeScript type context and give an example.

Think about how you can reuse a variable's type without rewriting it.

Describe the difference between typeof in JavaScript runtime and in TypeScript type context.

One is about values during running, the other about types during coding.