how to use the assignwith function from the lodash library in typescript

To use the assignWith() function from the Lodash library in TypeScript with type annotations, we first need to install the library using npm:

npm install lodash
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Then we import the function in our TypeScript code as follows:

index.ts
import { assignWith } from 'lodash';
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Next, we can use the assignWith() function to merge two objects and apply a customizer function for each key-value pairs using TypeScript. The function takes three parameters: the target object, the source object, and the customizer function. For example:

index.ts
interface User {
  name: string;
  age: number;
}

const user1: User = { name: 'John', age: 30 };
const user2: User = { name: 'Jane', age: 25 };

const customizer = (objValue: any, srcValue: any, key: string, object: any, source: any) => {
  // Return the value from the target object if the key is "name"
  return key === 'name' ? objValue : srcValue;
}

const mergedUser = assignWith({}, user1, user2, customizer);

console.log(mergedUser); // Output: { name: 'John', age: 25 }
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In this example, we define an interface User to specify the shape of our objects. We then create two objects of type User with different properties. We define a customizer function that takes two values and returns one of them depending on the key. We call the assignWith() function with an empty object {} as the first argument to avoid mutating the original objects, followed by the two objects to merge (user1 and user2) and the customizer function. The result is a merged object with the properties from both objects and the customizer applied to the name property.

Note that we can use TypeScript generics to specify the types of the target and source objects, which can help catch type errors at compile time:

index.ts
interface User {
  name: string;
  age: number;
}

const user1: User = { name: 'John', age: 30 };
const user2: User = { name: 'Jane', age: 25 };

const customizer = (objValue: any, srcValue: any, key: string, object: any, source: any) => {
  // Return the value from the target object if the key is "name"
  return key === 'name' ? objValue : srcValue;
}

const mergedUser = assignWith<User, User>({}, user1, user2, customizer);

console.log(mergedUser); // Output: { name: 'John', age: 25 }
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In this example, we use the generic types User to specify the types of the target and source objects. This can help catch type errors at compile time and make the code more robust.