Why This Is Asked

infer is an advanced conditional type feature that separates TypeScript power users from casual users. Interviewers use this to assess whether you can build utility types, understand conditional type mechanics, and read complex generic signatures in library code.

Key Concepts

infer declares a type variable within the extends clause of a conditional type, capturing a sub-type
The inferred variable is only available in the true branch of the conditional
Powers built-in utility types: ReturnType<T>, Parameters<T>, Awaited<T>, InstanceType<T>
Can extract types from any position: function return types, promise inner types, array element types, tuple positions
Advanced: multiple infer positions, recursive conditional types, template literal type extraction

Expected Answer

The infer keyword allows you to declare a type variable within the extends clause of a conditional type. This variable can then be used in the “true” branch of the condition to extract a specific type from a larger structure.

Practical Example: Unpacking a Promise

If you want to know what type a Promise resolves to:

type UnpackPromise<T> = T extends Promise<infer U> ? U : T;

type Result = UnpackPromise<Promise<string>>; // Result is 'string'
type NonPromise = UnpackPromise<number>; // NonPromise is 'number'

Advanced Usage: Function Return Types

You can use it to extract the return type of a function:

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

Why it’s useful

It allows for highly dynamic type transformations without having to explicitly pass every sub-type as a generic parameter. It is the basis for many built-in TypeScript utility types like ReturnType, Parameters, and Awaited.

Common Mistakes

Scope: Trying to use the inferred type in the false branch of the conditional (it’s only available in the true branch).
Multiple Inferences: Getting confused when inferring from multiple positions (e.g., inferring both the argument and return type of a function).

Follow-up Questions

How would you use infer to get the element type of an array? (Answer: T extends (infer U)[] ? U : T).
Can you use infer in template literal types? (Answer: Yes, for string manipulation).

The infer Keyword