TS2322Verified Production Fix

Type 'X' is not assignable to type 'Y' (TS2322)

Solve TypeScript TS2322 compilation error. Understand structural typing, excess property checks, optional chaining, and proper union narrowing.

Root Cause Mechanical Summary

A variable or function parameter was assigned a value whose structural type conflicts with the declared type contract. TypeScript’s static analyzer rejects the assignment to prevent runtime property access failures.

TypeScript relies on structural subtyping. If type A lacks required properties declared on type B, or property primitive types conflict (e.g. `string | null` assigned to `string`), TS2322 is emitted.

Vulnerable / Problematic Syntax

Before

// TS2322: Type 'string | null' is not assignable to type 'string'
function sendEmail(email: string) { /* ... */ }
const userEmail: string | null = getUserEmail();
sendEmail(userEmail);

Production-Safe Remediation

After

// Narrow the type before calling
function sendEmail(email: string) { /* ... */ }
const userEmail: string | null = getUserEmail();

if (userEmail !== null) {
  sendEmail(userEmail); // Narrowed to string safely
} else {
  // Handle fallback or throw error
}

Resolution Note: Use type guards (`if (val !== null)`) or nullish coalescing to satisfy non-nullable contracts.

Step-by-Step Triage Checklist

Examine the specific property identified in the compiler error diagnostic.
Check if `strictNullChecks` is enabled in `tsconfig.json`.
Use type narrowing or type assertions (`as Type`) only when runtime checks guarantee validity.

Typecheck Quality Gate in CI

.github/workflows/typecheck.yml

Prevent recurrence by enforcing this verification check in staging or pre-commit hooks:

name: Typecheck
on: [push, pull_request]
jobs:
  check:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - run: bun install
      - run: bun run typecheck

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JSON to Zod Schema

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Frequently Asked Questions

What is the difference between TS2322 and TS2345?: TS2322 is emitted on variable assignment (`const a: string = 123;`), while TS2345 is emitted when passing arguments to a function call.

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