TypeScript interview questions and quick answers

Use an indexed access with number: T[number]. For string[] it gives string, and for a readonly tuple declared as const it gives the union of its member literals.

An assertion function has a return type like asserts x is string. If it returns normally, TypeScript narrows x for the rest of the scope; if the check fails it must throw. It works like a guard that replaces an if block.

Awaited<T> gives the type you get after awaiting a promise, unwrapping nested promises too. Awaited<Promise<string>> is string. It is mostly used with ReturnType to get the resolved type of an async function.

A conditional type picks one of two types based on a check: T extends U ? X : Y. If T is assignable to U the result is X, otherwise Y. It lets types make decisions, like an if statement for types.

A discriminated union is a union of object types that share a literal-typed property, such as kind or type. Checking that property narrows the whole object to one member, so each branch sees only the fields that belong to it.

When a conditional type is applied to a naked type parameter that is a union, it runs once per union member and joins the results. IsString<string | number> becomes IsString<string> | IsString<number>. Wrapping in [T] turns this off.

"T[number] is the element type of any array-like type. With a const-asserted array, (typeof list)[number] gives the union of its literal members, so one array drives both runtime values and the type, without an enum."

An array type says "a list of something". To get that "something" you ask for the type at any numeric index: T[number]. Combine this with as const and a list of strings turns into a type with exactly those strings. One list then serves both the runtime loop and the type.