How to fix Mapped type property must be computed using 'in' syntax in TypeScript

First, examine your mapped type definition to ensure it uses the correct in syntax. The basic pattern is:

// Correct syntax
type MappedType<T> = {
  [K in keyof T]: T[K];
};

// Incorrect syntax (causes error)
type WrongType<T> = {
  [K: keyof T]: T[K];  // Error: Mapped type property must be computed using 'in' syntax
};

Make sure you're using in not : between the property variable and the key source.

Mapped types can only be used in type aliases, interfaces, and type parameters. They cannot be used in value positions or runtime code.

// Allowed: Type alias
type Readonly<T> = { readonly [K in keyof T]: T[K] };

// Allowed: Interface
interface ReadonlyInterface<T> {
  readonly [K in keyof T]: T[K];
}

// Not allowed: In function return type (without type alias)
function createReadonly<T>(obj: T) {
  return { readonly [K in keyof T]: T[K] }; // Error
}

If you need to use mapped types in function signatures, define them as separate type aliases first.

For advanced mapped types with modifiers or conditional logic, ensure the syntax is correct:

// Correct: With optional modifier
type Partial<T> = { [K in keyof T]?: T[K] };

// Correct: Removing readonly modifier
type Mutable<T> = { -readonly [K in keyof T]: T[K] };

// Correct: Key remapping with 'as' (TypeScript 4.1+)
type Getters<T> = {
  [K in keyof T as `get${Capitalize<string & K>}`]: () => T[K];
};

// Incorrect: Missing proper syntax
type Wrong<T> = {
  [K keyof T]?: T[K];  // Missing 'in' keyword
};

Pay special attention to:
- The in keyword after the property variable
- Proper use of keyof operator
- Correct placement of modifiers (?, readonly, -readonly, -?)
- Valid use of as clause for key remapping

Don't confuse mapped types with index signatures. They have different syntax and purposes:

// Index signature (for objects with dynamic keys)
interface StringDictionary {
  [key: string]: string;  // Uses colon (:)
}

// Mapped type (for transforming existing types)
type ReadonlyDictionary<T> = {
  [K in keyof T]: T[K];   // Uses 'in'
};

// Common mistake: mixing the syntax
type Wrong<T> = {
  [K: keyof T]: T[K];  // Error: This looks like an index signature but uses keyof
};

Remember:
- Index signatures use [key: type]: valueType syntax
- Mapped types use [K in keyof T]: transformedType syntax
- They serve different purposes and cannot be mixed

When in doubt, use the TypeScript Playground to test your mapped type syntax:

1. Go to https://www.typescriptlang.org/play
2. Paste your mapped type code
3. Check for errors in the editor
4. Experiment with different syntax variations

Example test:

// Test this in playground
type TestType<T> = {
  [K in keyof T]: T[K];
};

interface Example {
  name: string;
  age: number;
}

type TestResult = TestType<Example>;
// Should produce: { name: string; age: number; }

The playground provides instant feedback and helps you understand the correct syntax.

Some mapped type features require specific TypeScript versions:

- Basic mapped types: TypeScript 2.1+
- Conditional types with mapped types: TypeScript 2.8+
- Key remapping with as: TypeScript 4.1+
- Template literal types in mapped types: TypeScript 4.1+

Check your TypeScript version:

npx tsc --version
# or
npm list typescript

Update if needed:

npm install typescript@latest --save-dev

Ensure your tsconfig.json targets a compatible version and that your IDE uses the same TypeScript version as your project.