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TypeScript Error with Generics in Object Transformation Function

Learn why TypeScript generates an error when using generics in an object transformation function and how to properly implement type preservation.

#typescript-generics#object-transformation#type-preservation#object-fromentries#as-const-typescript
Question

Why does TypeScript generate an error when using generics in an object transformation function?

I wrote the following function to convert string values in an object to lowercase:

typescript
export function lowercasedObject<T extends { [k: string]: unknown }>(object: T): T {
  return Object.fromEntries(Object.entries(object).map(([key, value]) => [key, typeof value === 'string' ? value.toLowerCase() : value]))
}

However, TypeScript gives an error on the return line:

Type '{ [k: string]: unknown; }' is not assignable to type 'T'.
'{ [k: string]: unknown; }' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{ [k: string]: unknown; }'.ts(2322)

Why does this error occur and how do I properly implement an object transformation function while preserving its original type?

NeuroAgent

TypeScript generates this error due to strict type checking when working with generics. The type system cannot guarantee that the converted object maintains the exact structure of type T, even if it matches the constraint { [k: string]: unknown }.

Table of Contents

Error Cause

The error occurs because TypeScript cannot prove that the returned object exactly matches type T. Although Object.fromEntries() returns an object that matches the constraint { [k: string]: unknown }, TypeScript cannot guarantee that it maintains:

  1. The exact object keys
  2. The specific nested types
  3. The exact object structure

As explained in the TypeScript documentation, the compiler is concerned that T might be a specific subtype of the constraint that won’t exactly match the returned object.

Solution using as const

To solve this problem, you can use as const in combination with the correct return type:

typescript
export function lowercasedObject<T extends { [k: string]: unknown }>(object: T): T {
  const result = Object.fromEntries(
    Object.entries(object).map(([key, value]) => 
      [key, typeof value === 'string' ? value.toLowerCase() : value]
    )
  ) as const;
  
  // Force cast to type T
  return result as T;
}

However, this may still cause errors in strict mode. A more robust solution:

typescript
export function lowercasedObject<T extends Record<string, unknown>>(object: T): T {
  return Object.fromEntries(
    Object.entries(object).map(([key, value]) => 
      [key, typeof value === 'string' ? value.toLowerCase() : value]
    )
  ) as T;
}

Alternative Approaches

1. Using unknown and type checking

typescript
export function lowercasedObject<T extends Record<string, unknown>>(object: T): T {
  const result: Record<string, unknown> = {};
  
  for (const [key, value] of Object.entries(object)) {
    result[key] = typeof value === 'string' ? value.toLowerCase() : value;
  }
  
  return result as T;
}

2. Recursive approach for nested objects

If you need to handle nested objects:

typescript
export function lowercasedObject<T extends Record<string, unknown>>(object: T): T {
  const result: Record<string, unknown> = {};
  
  for (const [key, value] of Object.entries(object)) {
    if (typeof value === 'string') {
      result[key] = value.toLowerCase();
    } else if (value && typeof value === 'object' && !Array.isArray(value)) {
      result[key] = lowercasedObject(value as Record<string, unknown>);
    } else {
      result[key] = value;
    }
  }
  
  return result as T;
}

3. Using const assertion on the input object

You can require the user to use as const:

typescript
export function lowercasedObject<T extends readonly { readonly [k: string]: unknown }>(object: T): T {
  return Object.fromEntries(
    Object.entries(object).map(([key, value]) => 
      [key, typeof value === 'string' ? value.toLowerCase() : value]
    )
  ) as T;
}

// Usage:
const obj = { name: 'Hello', age: 25 } as const;
const result = lowercasedObject(obj); // Type is preserved

Complete Working Example

typescript
// Main implementation
export function lowercasedObject<T extends Record<string, unknown>>(object: T): T {
  return Object.fromEntries(
    Object.entries(object).map(([key, value]) => 
      [key, typeof value === 'string' ? value.toLowerCase() : value]
    )
  ) as T;
}

// Usage examples
interface User {
  name: string;
  age: number;
  isActive: boolean;
}

const user: User = {
  name: 'JOHN DOE',
  age: 30,
  isActive: true
};

const processedUser = lowercasedObject(user);
// processedUser has type User
// { name: 'john doe', age: 30, isActive: true }

// With arrays
interface DataPoint {
  id: string;
  value: number;
  metadata: {
    source: string;
    tags: string[];
  };
}

const dataPoint: DataPoint = {
  id: '123',
  value: 42,
  metadata: {
    source: 'API',
    tags: ['IMPORTANT', 'TEST']
  }
};

const processedData = lowercasedObject(dataPoint);
// processedData has type DataPoint

Conclusion

  1. The error occurs due to strict typing - TypeScript cannot guarantee maintaining the exact structure of type T when converting an object.

  2. Main solutions:

    • Using as T for type assertion
    • Working with Record<string, unknown> as an intermediate type
    • Requiring as const from the user for better typing

  3. For complex scenarios:

    • Recursive processing of nested objects
    • Combining multiple approaches
    • Using conditional types for more precise typing

  4. Recommendation - in most cases, the basic solution with as T is sufficient, but for complex structures, a more detailed approach to type handling may be required.

Sources

  1. TypeScript: Documentation - Generics
  2. Stack Overflow: TypeScript Generics: ‘type is not assignable to type T’
  3. Stack Overflow: Generic return type in object transformation
  4. GitHub: Type ‘T’ is not assignable to type when Generic Anonymous Function is inside an interface
  5. PersistDev: Exploring Generic Inference with Object Arguments in TypeScript
How to implement recursive transformation of nested objects in TypeScript with type preservation?What is the difference between using Record<string, unknown> and { [k: string]: unknown } in TypeScript?How to handle arrays in objects when transforming with generics in TypeScript?Why can't TypeScript infer types when using Object.fromEntries with generics?What are the best practices for working with generics in TypeScript for object operations?
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