TS2322: Type 'X' is not assignable to type 'Y'

Common causes

• Assigning a string to a variable declared as a number, or vice versa
• A function's return value doesn't match its declared return type
• Passing a union type where only one specific member is accepted
• Assigning null or undefined to a non-nullable type with strict null checks enabled

How to fix it

1. Update the value to match the expected type, or loosen the declared type to accept the actual value
2. If the type annotation is wrong, fix it: change number to string if the value is always a string
3. Use a type guard to narrow before assigning: if (typeof value === 'string') { ... }
4. As a last resort, use a type assertion: value as TargetType — only when you're certain of the type

Example

Assigning a string literal to a variable declared as number