Generic Basics

Preview — 3 of 10 questions

What is the type of result here?

javascript

function identity<T>(value: T): T {
  return value;
}

const result = identity("hello");

Aunknown

Bstring — TypeScript infers T = string from the argument

Cany

Which of these is the correct way to type a function that returns the first element of any array?

javascript

// Option A:
function first(arr: any[]): any { return arr[0]; }

// Option B:
function first<T>(arr: T[]): T { return arr[0]; }

// Option C:
function first(arr: unknown[]): unknown { return arr[0]; }

AOption A — any[] is most flexible

BOption B — generic preserves the element type

COption C — unknown is safest

DAll three are equivalent

What does TypeScript infer for T and U?

javascript

function pair<T, U>(first: T, second: U): [T, U] {
  return [first, second];
}

const result = pair("hello", 42);
// result: ?

A[T, U]

B[any, any]

C[string, number]

Dstring | number

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