Decorators & Project Setup

Preview — 3 of 10 questions

What does this strict flag add?

javascript

// tsconfig: "noUncheckedIndexedAccess": true

const arr: string[] = ["a", "b", "c"];
const first = arr[0]; // Type of first?

const obj: Record<string, number> = { a: 1 };
const val = obj["b"]; // Type of val?

Afirst: string, val: number — same as without the flag

BCompile error — indexed access is disallowed with this flag

Cfirst: string, val: number | undefined — only object access adds undefined

Dfirst: string | undefined, val: number | undefined — index access always adds | undefined

What's different with exactOptionalPropertyTypes?

javascript

// tsconfig: "exactOptionalPropertyTypes": true

interface Config {
  theme?: 'light' | 'dark';
}

const c1: Config = { theme: 'light' };   // (A) ?
const c2: Config = { theme: undefined }; // (B) ?
const c3: Config = {};                    // (C) ?

A(A) and (C) are valid; (B) errors — undefined is not the same as absent

BAll three are valid

C(A) is valid; (B) and (C) error

DAll three error — optional properties need | undefined in the type

What error does this flag catch?

javascript

// tsconfig: "noImplicitReturns": true

function getLabel(status: 'active' | 'inactive' | 'pending'): string {
  if (status === 'active') return 'Active';
  if (status === 'inactive') return 'Inactive';
  // 'pending' case missing!
}

AError — not all code paths return a value (function lacks a final return)

BNo error — TypeScript infers the function can return undefined

CError — the return type should be string | undefined

DError only if the missing case is reachable based on types

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