Union & Intersection Types — Series 3

Preview — 3 of 10 questions

javascript

interface Form {
  email: string;
  phone?: string;
}

type Draft = Partial<Form>;

const d: Draft = {};

ACompile-time error — Partial cannot be applied to a type that already has optional properties

BCompiles fine — Draft is { email?: string; phone?: string }, so the empty object is valid

CCompiles fine, but Draft is { email?: string; phone: string } — Partial toggles each modifier

DCompile-time error — email is still required, because Partial only affects the last property

javascript

const mutable: string[] = ["a", "b"];

const ro: readonly string[] = mutable;
const back: string[] = ro;

ABoth compile — readonly is erased, so the two types are identical

BBoth fail — a mutable array and a readonly array are unrelated types

Cro fails; back compiles

Dro compiles; back is a compile-time error — a readonly string[] is missing the mutating methods that string[] requires

javascript

function total(input: number | number[]): number {
  if (typeof input === "number") {
    return input;
  }
  return input.reduce((a, b) => a + b, 0);
}

ACompiles fine — typeof input === "number" narrows to number in the branch, and to number[] after it

BCompile-time error — typeof reports "object" for arrays, so the union is never narrowed

CCompile-time error on input.reduce — input is still number | number[] after the if

DCompiles, but input.reduce is typed any because the array's element type is unknown

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