ts/strict

Why

Without strictNullChecks, every value may silently be null or undefined and the compiler never says so; without noImplicitAny, untyped parameters turn checking off where it matters most. Most "Cannot read properties of undefined" errors in production are what strict mode reports at build time.

Fix

Remove "strict": false and any strict flag set to false. TypeScript 6.0 and later turn strict on by default; on 5.x write "strict": true. In a large old codebase, turn it on and silence the existing errors file by file with // @ts-expect-error and a reason, so new code is strict from today.

Examples

// bad
{ "compilerOptions": { "strict": false } }
{ "compilerOptions": { "strict": true, "strictNullChecks": false } }
// good
{ "compilerOptions": { "strict": true, "noUncheckedIndexedAccess": true } }

Sources

• TypeScript, TSConfig reference (strict)
• TypeScript 6.0 release notes (strict is now true by default)

Read it in the terminal

npx github:dawidolko/frontend-skills explain ts/strict