Compiler API & References — Series 2

Preview — 3 of 10 questions

Project structure: src/index.ts, src/utils/helper.ts, and a stray scripts/build.ts (outside src/) also included via include.

javascript

{
  "compilerOptions": {
    "outDir": "./dist",
    "rootDir": "./src"
  }
}

ACompile-time error — rootDir requires every input file (including scripts/build.ts) to be located under it; since scripts/build.ts falls outside src/, tsc rejects the configuration

BCompiles fine — outDir and rootDir only affect where individual files are emitted, never which files are allowed as input

CCompiles fine, and scripts/build.ts is simply silently skipped during compilation

DCompiles fine, and TypeScript automatically adjusts rootDir behind the scenes to also include scripts/build.ts

javascript

class Base {
  greeting = 'hello';
}
class Derived extends Base {
  greeting = this.computeGreeting();
  computeGreeting() { return 'hi'; }
}

ANo — useDefineForClassFields only affects #-prefixed private fields, never regular public ones

BYes — with useDefineForClassFields: true (the default once target is ES2022+), class fields are initialized following Object.defineProperty-based semantics, matching the true ECMAScript class fields specification; this can subtly change field-initialization timing and interaction with the constructor chain compared to the older, simpler assignment-based (this.field = value) emulation TypeScript used when the flag is false

CNo — this flag only controls whether class fields are emitted as documentation comments in the compiled output

DYes, but the difference only ever matters for static class fields, never instance fields

With preserveConstEnums: true, what changes compared to the default const enum behavior?

javascript

const enum Direction { Up, Down }
export { Direction };

Aconst enum members can no longer be used inside switch statements

BAll references to enum members are still inlined as literal values everywhere, exactly like a normal const enum — nothing observable changes

CTypeScript still inlines direct references to Direction's members at their use sites (the usual const enum optimization), but additionally emits a real runtime Direction object — useful for scenarios where the enum needs to be importable and inspectable by other, separately-compiled files (such as under isolatedModules-style single-file transpilation) that can't see the inlining information the full-program compiler has

DCompile-time error — preserveConstEnums can't be combined with exporting a const enum

Answer all 10 questions (7 more), see explanations for every answer, and track your score.