How does TypeScript handle strict null checks?

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Accepted Answer

// strictNullChecks: false (old behavior)
let name: string = null;      // ✅ no error — dangerous!
let age: number = undefined;  // ✅ no error — dangerous!
let user: object = null;      // ✅ no error

// This compiles fine but crashes at runtime
function greet(name: string) {
  return name.toUpperCase(); // 💥 runtime crash if null passed
}

greet(null); // ✅ no compile error, 💥 runtime error // strictNullChecks: true
let name: string = null;      // ❌ Error — null not assignable to string
let age: number = undefined;  // ❌ Error — undefined not assignable to number

// null and undefined are their own types now
let a: null = null;           // ✅
let b: undefined = undefined; // ✅

// Must explicitly allow null
let name: string | null = null; // ✅ union type // tsconfig.json
{
  "compilerOptions": {
    "strictNullChecks": true,  // enable individually

// OR enable all strict checks at once (recommended)
    "strict": true             // includes strictNullChecks + more
  }
}

How does TypeScript handle strict null checks?

Strict null checks (strictNullChecks) is a TypeScript compiler option that makes null and undefined their own distinct types — preventing them from being assigned to other types unless explicitly allowed.

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