Checkpoint: Shipping TypeScript

Five questions on this section. Answer all of them, then see your score and why each answer is right or wrong.

A script run with node checkout.ts (Node’s built-in type stripping) contains enum Channel { Web = 'web' } and fails at startup. Which compiler flag would have flagged the enum during tsc?

isolatedModules
verbatimModuleSyntax
erasableSyntaxOnly
noEmit

Why can this charge run while the cart is unlocked?

TypeScript

async function pay(cartId: string, needsLock: boolean) {
  if (needsLock) {
    using lock = acquireCartLock(cartId);
  }
  await charge(cartId);
}

using is not allowed in async functions.
It should be await using, because the function is async.
The lock is never released.
The lock is disposed at the end of the if block, before charge runs.

interface Window { dataLayer: unknown[] } sits at the top level of src/analytics.ts, which also has imports. window.dataLayer is still an error. Why?

In a module file, a top-level interface is local to that module; wrap it in declare global { … } to merge with the global Window.
Window is a built-in type and cannot be extended.
It must be a type alias instead of an interface.
Interface merging only works in .d.ts files.

A monorepo has six packages type-checked as one big program, and every small change re-checks all of them for 90 seconds. Which change helps most?

Turn off strict in the slowest package.
Split it into project references (composite: true per package, tsc -b at the root), so unchanged packages are not re-checked.
Set skipLibCheck: false to catch more problems early.
Merge all packages into one src folder.

A .js file starts with // @ts-check and declares /** @param {Order} order */ function ship(order) { … }. What does TypeScript do with it (with allowJs on)?

Nothing, until the file is renamed to .ts.
It checks the file only if checkJs is also true in tsconfig.
It checks only the JSDoc comments, not the code.
It type-checks the whole file, using the JSDoc comments as the types of ship and its parameter.

5 questions unanswered