TS2322: Type 'null' is not assignable to type 'string'

What’s happening?

This error occurs when a value that may be null is assigned to a variable that expects a strict type (like string).

With strictNullChecks enabled, TypeScript treats null as a distinct type.

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Reproduction

function getStoredValue(): string | null {
  const stored = localStorage.getItem('username');
  return stored; // Can be null
}

const username: string = getStoredValue();
// ❌ Type 'null' is not assignable to type 'string'

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Solutions

Option 1: Nullish coalescing operator

const username: string = getStoredValue() ?? "default"; // ✅

Option 2: Type narrowing

const value = getStoredValue();
if (value !== null) {
  const username: string = value; // ✅
}

Option 3: Update the type

const username: string | null = getStoredValue(); // ✅

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Common scenarios

localStorage

const token: string = localStorage.getItem('token');
// ❌ Type 'string | null' is not assignable

// ✅ Fix:
const token: string = localStorage.getItem('token') ?? "";

Database queries

const user: User = await db.findOne({ id: 1 });
// ❌ Type 'User | null' is not assignable

// ✅ Fix:
const user = await db.findOne({ id: 1 });
if (user === null) {
  throw new Error("User not found");
}

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Key takeaways

• null and undefined are different types in TypeScript
• Use ?? "" for default values
• Use type guards (!== null) for safety
• Avoid ! unless absolutely certain

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Additional resources

• strictNullChecks