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JavaScript

What is a Type X is not assignable to type Y?

← Errors

This TypeScript compile error (TS2322) is reported when you try to assign a value of one type to a variable, parameter, or return type that expects a different, incompatible type. TypeScript's structural type system checks that the shape of the value matches what is expected.

Why it happens

  • •Assigning the return value of a function to a variable declared with an incompatible type.
  • •Passing an argument of the wrong type to a function parameter.
  • •Returning a value from a function whose declared return type does not match.
  • •Narrowing a union type incorrectly, leaving a type branch that does not satisfy the target type.

Examples

JavaScript

Code that triggers the error

function greet(name: string): string {
  return "Hello, " + name;
}

const result: number = greet("Alice");

Error output

error TS2322: Type 'string' is not assignable to type 'number'.

How to fix it

Align the type of the value with the type of the target. Change the variable's type annotation, update the function's return type, or convert the value. Avoid using type assertions (as) to silence this error: fix the types instead. Use union types (string | null) when a value can legitimately be one of several types.

Fixed code

function greet(name: string): string {
  return "Hello, " + name;
}

const result: string = greet("Alice");
console.log(result);  // Hello, Alice

Related errors

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