For Conditional Types & infer, keep the contract between callers and implementations minimal. Consider generic constraints, variance, readonly data, and structural compatibility without hiding uncertainty behind assertions.
Build a Complete Mental Model
For Conditional Types & infer, keep the contract between callers and implementations minimal. Consider generic constraints, variance, readonly data, and structural compatibility without hiding uncertainty behind assertions.
Apply It in Production TypeScript
Type-check an original Conditional Types & infer example in strict mode, then test valid values, null/undefined, malformed API data, a new union variant, and module boundaries. Fix the contract or validation instead of silencing errors with assertions or any.
After This Lesson
type AwaitedResult<T> = T extends Promise<infer Value> ? Value : T;
type ApiResult = AwaitedResult<Promise<{ id: string }>>;
const item: ApiResult = { id: 'task-1' };ApiResult resolves to the promise's value type.Try It Yourself
Type-check an original Conditional Types & infer example in strict mode, then test valid values, null/undefined, malformed API data, a new union variant, and module boundaries. Fix the contract or validation instead of silencing errors with assertions or any.
Type Safety Warning
Assuming a successful TypeScript compile proves API data and runtime behavior are valid.
Conditional Types — TypeScript