A generic captures relationships between inputs and outputs. Constraints permit required operations without discarding the caller’s specific type.
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: A function constrained to {id: string} can return the exact subtype it received. 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: Find the relationship
The caller needs the same record type back, including extra fields. A helper returning only an ID-bearing interface loses that information.
Step 2: Constrain required operations
Use T extends a minimum shape so the implementation can read id. Do not invent fields that the constraint does not guarantee.
Step 3: Preserve the caller’s type
Return T when the actual original record is returned. Constructing a smaller object is not enough to satisfy every possible subtype.
Worked scenario
A function constrained to {id: string} can return the exact subtype it received.
function requireId<T extends { id: string }>(record: T): T {
if (record.id.length === 0) throw new Error('Missing ID');
return record;
}
const user = requireId({ id: 'u1', name: 'Ada' });
console.log(user.name); // name remains availableThe constraint enables the id check; T preserves name. Returning { id: record.id } would break the promise made to callers with additional required fields.
Common mistake
An unconstrained type parameter does not guarantee properties exist.
Verify the behavior
Pass records with different extra fields and a record missing id. Check inferred return types and the runtime empty-ID path separately.
Interview exercise
Preserve additional fields in a helper.
Answer and reasoning
Use a constrained T and return T; returning only the constraint would lose subtype information.
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.