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Why TypeScript throws an error with generics

Learn why TypeScript throws an error when using generics in object transformation functions. Complete guide to solving the problem while preserving types.

#object-fromentries#object-structure-preservation#type-assertion#type-conversion#typescript-generics#utility-types
Question

Why does TypeScript give an error when using such a generic?

I wrote a simple 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)

What is the problem and how can I properly implement such a function using generics in TypeScript?

NeuroAgent

The problem occurs because TypeScript cannot guarantee that an object created with Object.fromEntries() preserves the original type structure of object T. This error arises from TypeScript’s strict type system, which prevents potential type mismatches.

Contents

The Main Problem

The error Type '{ [k: string]: unknown; }' is not assignable to type 'T' occurs because TypeScript cannot prove that the object returned from Object.fromEntries() exactly matches type T.

Your code converts object T to an array of key-value pairs using Object.entries(), then creates a new object using Object.fromEntries(). The problem is that Object.fromEntries() returns type { [k: string]: unknown }, which is more generic than the original type T.

Why TypeScript Throws This Error

This error is part of TypeScript’s type system protection. The message T could be instantiated with a different subtype of constraint means that:

  1. You have a generic T with constraint extends { [k: string]: unknown }
  2. TypeScript cannot guarantee that the object created by Object.fromEntries() will have exactly the same structure as the original object
  3. For example, if T has specific property types (like name: string and age: number), then Object.fromEntries() will return an object with type { [k: string]: unknown }, which doesn’t preserve these specific types

Solutions

1. Using type assertion (not recommended)

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]
    )
  ) as T;
}

This approach works but disables type checking and can lead to runtime errors.

2. Using a more precise 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]
    )
  );
  return result as unknown as T;
}

3. Using a recursive type to preserve structure

typescript
type LowercasedObject<T extends { [k: string]: unknown }> = {
  [K in keyof T]: T[K] extends string ? Lowercase<T[K]> : T[K];
};

export function lowercasedObject<T extends { [k: string]: unknown }>(
  object: T
): LowercasedObject<T> {
  const result: { [k: string]: unknown } = {};
  for (const key in object) {
    if (Object.prototype.hasOwnProperty.call(object, key)) {
      const value = object[key];
      result[key] = typeof value === 'string' ? value.toLowerCase() : value;
    }
  }
  return result as LowercasedObject<T>;
}

Optimal Solution Using Generics

The best solution is to create a specialized type that preserves the object structure while transforming only string values:

typescript
type LowercasedObject<T extends { [k: string]: unknown }> = {
  [K in keyof T]: T[K] extends string ? Lowercase<T[K]> : T[K];
};

export function lowercasedObject<T extends { [k: string]: unknown }>(
  object: T
): LowercasedObject<T> {
  const result: { [k: string]: unknown } = {};
  for (const key in object) {
    if (Object.prototype.hasOwnProperty.call(object, key)) {
      const value = object[key];
      result[key] = typeof value === 'string' ? value.toLowerCase() : value;
    }
  }
  return result as LowercasedObject<T>;
}

This solution:

  1. Preserves the original object structure
  2. Transforms only string values to lowercase
  3. Preserves types of non-string values
  4. Correctly handles all object keys

Alternative Approaches

1. Using utility types

typescript
type Lowercased<T> = T extends string ? Lowercase<T> : T;

type LowercasedObject<T> = {
  [K in keyof T]: Lowercased<T[K]>;
};

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

2. Recursive transformation for nested objects

If you need to handle nested objects:

typescript
type LowercasedRecursive<T> = T extends string
  ? Lowercase<T>
  : T extends object
  ? {
      [K in keyof T]: LowercasedRecursive<T[K]>;
    }
  : T;

export function lowercasedObject<T extends Record<string, unknown>>(
  object: T
): LowercasedRecursive<T> {
  const result: { [k: string]: unknown } = {};
  for (const key in object) {
    if (Object.prototype.hasOwnProperty.call(object, key)) {
      const value = object[key];
      result[key] = typeof value === 'string' 
        ? value.toLowerCase() 
        : typeof value === 'object' && value !== null
        ? lowercasedObject(value as Record<string, unknown>)
        : value;
    }
  }
  return result as LowercasedRecursive<T>;
}

Practical Examples

Example 1: Basic usage

typescript
interface User {
  name: string;
  age: number;
  email: string;
}

const user: User = {
  name: 'JOHN DOE',
  age: 30,
  email: 'JOHN@EXAMPLE.COM'
};

const lowercasedUser = lowercasedObject(user);
// lowercasedUser will be of type:
// {
//   name: "john doe";
//   age: 30;
//   email: "john@example.com";
// }

Example 2: Mixed types

typescript
const mixedObject = {
  title: 'HELLO WORLD',
  count: 42,
  isActive: true,
  tags: ['TYPESCRIPT', 'JAVASCRIPT']
};

const lowercasedMixed = lowercasedObject(mixedObject);
// Result preserves all original types:
// {
//   title: "hello world";
//   count: 42;
//   isActive: true;
//   tags: ['TYPESCRIPT', 'JAVASCRIPT'];
// }

Example 3: Handling nested objects

typescript
interface Config {
  app: {
    name: string;
    version: string;
  };
  database: {
    host: string;
    port: number;
  };
}

const config: Config = {
  app: {
    name: 'MY APP',
    version: '1.0.0'
  },
  database: {
    host: 'localhost',
    port: 5432
  }
};

const lowercasedConfig = lowercasedObject(config);
// Nested objects will also be processed

Sources

  1. TypeScript: Documentation - Generics - Official documentation on TypeScript generics
  2. TypeScript Generic Constraints - Detailed explanation of generic constraints
  3. TypeScript Function with generics & Constraints in typescript - Practical examples of using generics
  4. typescript - Generic return type in object transformation - Solving return type issues in objects
  5. How To Use Generics in TypeScript - Tutorial on using generics
  6. Understanding TypeScript Generics - Deep understanding of generics
  7. Exploring the Power of TypeScript Generics - Advanced techniques for working with generics

Conclusion

The TypeScript error problem with generics occurs due to the strict type system that doesn’t allow automatic type conversion between similar structures. Key takeaways:

  1. Error cause: TypeScript cannot guarantee that Object.fromEntries() preserves the original type structure of object T

  2. Best solution: Use a specialized type LowercasedObject<T> that explicitly defines how types should be transformed

  3. Benefits of the correct approach:

    • Preserves type safety
    • Correctly handles mixed object types
    • Supports nested structures
    • Provides clear expectations from the function

  4. Practical recommendations:

    • Always use explicit return types for complex generics
    • Avoid type assertion when possible
    • Use utility types for transformations
    • Test the function with various object types

Proper implementation of generics in TypeScript requires understanding of the type system, but ensures maximum type safety and code reliability.

How to properly use recursive types in TypeScript for processing nested objects?What is the difference between type assertion and explicit type casting in TypeScript?How to create a utility type to convert all string values in an object to lowercase?Why can't TypeScript automatically infer types when using Object.fromEntries?How to handle complex data types when transforming objects in TypeScript?What are the best practices for working with generics in TypeScript?
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