Inferring types with infer keyword in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the infer keyword do in TypeScript?

The infer keyword lets TypeScript guess a type inside a conditional type. It helps extract or infer a type from another type.

intermediate

How do you use infer inside a conditional type?

You write a conditional type with extends and inside it use infer T to capture a type part. For example: type GetReturn<T> = T extends (...args: any[]) => infer R ? R : never;

beginner

What happens if the type does not match the pattern when using infer?

If the type does not match the pattern in the conditional type, the infer part is ignored and the false branch of the conditional type is used.

intermediate

Give an example of inferring the argument type of a function using infer.

Example: type ArgType<T> = T extends (arg: infer A) => any ? A : never; This extracts the type of the first argument of a function type T.

beginner

Why is infer useful in TypeScript?

infer helps create flexible and reusable types by extracting parts of complex types automatically. It reduces manual type writing and improves type safety.

What does the infer keyword do in TypeScript conditional types?

A Extracts a type from another type

B Declares a new variable

C Creates a new interface

D Defines a function

infer is used to extract or guess a type inside conditional types.

Which syntax correctly uses infer to get a function's return type?

A type R<T> = T => infer R;

B type R<T> = T extends (...args: any[]) => infer R ? R : never;

C type R<T> = T extends infer R ? R : never;

D type R<T> = infer R extends T ? R : never;

Option B correctly uses infer inside a conditional type to extract the return type.

What happens if the type does not match the pattern when using infer?

A The program crashes

B TypeScript throws an error

C The inferred type becomes <code>any</code>

D The <code>infer</code> type is ignored and the false branch is used

If the pattern does not match, the conditional type uses the false branch, ignoring infer.

Which of these extracts the first argument type of a function using infer?

A type Arg<T> = infer A extends T ? A : never;

B type Arg<T> = T extends infer A => any ? A : never;

C type Arg<T> = T extends (arg: infer A) => any ? A : never;

D type Arg<T> = T => infer A;

Option C correctly infers the first argument type of a function type.

Why is infer useful in TypeScript?

A It helps extract types automatically for flexible type definitions

B It runs code at runtime

C It creates new variables

D It replaces interfaces

infer helps create reusable and safe types by extracting parts of types automatically.

Explain how the infer keyword works inside a TypeScript conditional type.

Think about how TypeScript guesses a type inside a condition.

Give an example of using infer to extract the return type of a function type.

Use a conditional type with <code>extends</code> and <code>infer R</code>.