tsc TS2322: Type 'X' is not assignable to type 'Y' in CI

By Daniel Zoghalchali·Latchkey

TS2322 is an assignment-compatibility error: the type on the right of an assignment (or a property value, or a returned value) is not a subtype of the declared target type, so tsc rejects it.

What this error means

tsc fails with "error TS2322: Type 'string' is not assignable to type 'number'" (or similar), often with a follow-on note about which property or member caused the mismatch.

src/config.ts:4:3 - error TS2322: Type 'string' is not assignable to type 'number'.

4   port: '8080',
    ~~~~

Common causes

The value type does not match the declared type

A string is assigned where a number is declared, or a wider union is assigned to a narrower one. tsc enforces structural assignability and rejects it.

A widened literal or a missing property in an object literal

Object literals are checked exactly; an extra or wrongly typed property makes the literal not assignable to the target interface.

How to fix it

Make the value match the target type

1. Read the "Type X is not assignable to type Y" pair to see the exact mismatch.
2. Convert or correct the value so its type is a subtype of the target.
3. Re-run tsc to confirm the assignment now type-checks.

// '8080' is a string; the field expects a number
const config = { port: 8080 };

Widen the target type if the broader type is intended

If both values are legitimately allowed, declare the target as a union rather than forcing a cast.

type Port = number | `${number}`;

How to prevent it

• Type config and API shapes explicitly so mismatches surface at the source.
• Avoid as casts that paper over real assignability errors.
• Run tsc locally before pushing so TS2322 is caught before CI.

Frequently asked questions

Related guides

References

• Node.js documentation
• npm CLI documentation
• GitHub Actions documentation