Infer introduces a type variable inside a conditional pattern. It extracts structure rather than executing a function or inspecting runtime values.
Before you start
You should be comfortable with JavaScript values, functions and objects. Use a TypeScript project with strict checking enabled when trying the examples. Separate what the compiler proves from what must still be checked when values arrive at runtime.
The practical goal is to reason through this situation: Match a function type to extract its return type or a promise-like structure to extract a value type. Read the walkthrough first, then try the interview exercise before opening its answer. The important part is explaining the decision and its consequences, rather than remembering a definition alone.
Step-by-step walkthrough
Step 1: Describe the matching structure
Infer names part of a conditional type pattern. It extracts an output type from a callable shape rather than invoking the function.
Step 2: Choose a standard utility
ReturnType and Awaited already express common function and promise extraction. Prefer those utilities when they match the intended behavior.
Step 3: Verify ambiguous signatures
Overloads and unions require explicit inspection. Do not assume extracting a type models every possible runtime call or its argument relationship.
Worked scenario
Match a function type to extract its return type or a promise-like structure to extract a value type.
async function loadCount() { return 4; }
type Pending = ReturnType<typeof loadCount>; // Promise<number>
type Count = Awaited<Pending>; // number
type ExtractReturn<T> = T extends (...args: never[]) => infer R
? R : never;
type Example = ExtractReturn<() => string>; // stringReturnType captures the declared function result; Awaited unwraps the asynchronous value model. Neither utility starts a request or observes an actual resolved response.
Common mistake
Overload extraction and unions have rules that may differ from intuition.
Verify the behavior
Check synchronous, asynchronous and non-function inputs. Add an overloaded function fixture before using extraction to define a public overload-dependent API.
Interview exercise
Unwrap an async function result.
Answer and reasoning
Combine ReturnType and Awaited; verify overload behavior instead of assuming every call signature is represented.
Continue learning
Compare the scenario with the TypeScript interview questions and test your understanding with the TypeScript MCQs. For terminology and implementation details, consult the reference material.