Strict Mode

"Strict mode" is the recommended TypeScript baseline. "strict": true in tsconfig flips on several smaller checks at once.

What it turns on

Bonus flags worth adding

{
    "compilerOptions": {
        "strict": true,
        "noUncheckedIndexedAccess": true,
        "exactOptionalPropertyTypes": true,
        "noImplicitOverride": true,
        "noFallthroughCasesInSwitch": true,
        "noImplicitReturns": true,
        "noUnusedLocals": true,
        "noUnusedParameters": true,
        "noPropertyAccessFromIndexSignature": true
    }
}

Why bother

• Most "TypeScript didn't help me here!" stories trace back to flags that should have been on.
• The compile-time cost of strict mode is small; the runtime cost of missing these bugs is large.
• Code review can focus on logic, not "is this null when it shouldn't be?".

Adopting strict in an existing project

Don't try to turn everything on at once. Phase it:

1. Add "strict": true to a new sub-tsconfig used only by new code.
2. Migrate files one at a time — TS shows you what to fix.
3. Optional: install ts-strict-migrate to track per-file progress.
4. Once everything passes, flip the root tsconfig.

Example

// In strict mode, these become errors:
// - implicit any
// - null / undefined slipping through
// - this in a function not bound
// - returning undefined from a non-void function
//
// Turn it on:  "strict": true  in tsconfig.json.
console.log('Strict mode is the recommended default');

Exercise

Test yourself