typescript-advanced-types

Define a type parameter like `function identity<T>(value: T): T` and constrain it with `extends` when needed, such as `T extends HasLength`. TypeScript infers the type argument from the call site, preserving type safety without explicit annotations.

Use a conditional type with infer: `type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never`. Applying it to `typeof myFunction` yields the exact return type, which stays in sync when the function changes.

Mapped types iterate over an existing type's properties to transform them, like `Partial<T>` or key remapping with `as`. Conditional types select a type based on a check, like `T extends string ? A : B`, and distribute over unions.

Use `unknown` for values whose type is not yet known, because it forces type narrowing through guards before use. `any` disables all type checking and should be avoided since it defeats TypeScript's compile-time safety.

Each conditional branch multiplies the type-checking work the compiler performs, and recursive types compound this cost. Cache complex type computations in named aliases, limit recursion depth, and prefer simpler types where possible.

Write compile-time type tests using an assertion helper like `type AssertEqual<T, U> = [T] extends [U] ? ([U] extends [T] ? true : false) : false`. Assign the result to a type alias; mismatches surface as type errors during compilation.