Lesson 26 / 26

Strict Mode & Compiler Errors

Q: What does `"strict": true` in tsconfig.json do?

Bundles several strict type-checking options together. strict is a shorthand that turns on strictNullChecks, noImplicitAny, and several other checks at once.

One flag that turns on TypeScript's full safety net, and how to read what it tells you.

The strict flag

"strict": true in tsconfig.json bundles several checks — strictNullChecks, noImplicitAny, and more — into one switch. New projects should turn it on from day one.

strictNullChecks catch

With strict null checks, null/undefined aren't silently allowed everywhere — you must handle them explicitly.

function getLength(s: string | null): number {
  return s.length;
  // Error: Object is possibly 'null'.
}

function getLengthSafe(s: string | null): number {
  return s ? s.length : 0; // narrowed, OK
}

Reading compiler errors

TS error messages name the exact incompatible types ("Type 'X' is not assignable to type 'Y'") — read the two type names first; they usually point straight at the fix.

Quick check: What does `"strict": true` in tsconfig.json do?

Enables a single extra check
Bundles several strict type-checking options together
Only affects the output file names

Answer

Bundles several strict type-checking options together — strict is a shorthand that turns on strictNullChecks, noImplicitAny, and several other checks at once.