Declaration of custom object or type in Node.js

In Node.js, when you want to assign one object to another that is declared by a custom type or class, you typically need to ensure compatibility between the source object's structure and the target class's expectations. Here's a breakdown:

Scenario 1: Assigning Plain Objects to Class Instances

If you have a custom class and want to assign properties from a plain object to an instance of that class, you can use one of the following methods:

Example:

class CustomType {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }
}

// Plain object
const plainObject = { name: "John", age: 30 };

// Assign using Object.assign()
const customInstance = new CustomType();
Object.assign(customInstance, plainObject);

console.log(customInstance instanceof CustomType); // true
console.log(customInstance); // CustomType { name: 'John', age: 30 }

Scenario 2: Using a Constructor

You can modify your class to accept an object in the constructor for direct assignment:

class CustomType {
  constructor({ name, age }) {
    this.name = name;
    this.age = age;
  }
}

// Plain object
const plainObject = { name: "John", age: 30 };

// Create an instance
const customInstance = new CustomType(plainObject);

console.log(customInstance instanceof CustomType); // true
console.log(customInstance); // CustomType { name: 'John', age: 30 }

Scenario 3: Validation or Transformation

If you need to validate or transform the input object before assigning, you can implement a custom method:

class CustomType {
  constructor() {
    this.name = "";
    this.age = 0;
  }

  assignFromObject(obj) {
    if (obj.name && typeof obj.name === "string") {
      this.name = obj.name;
    }
    if (obj.age && typeof obj.age === "number") {
      this.age = obj.age;
    }
  }
}

// Plain object
const plainObject = { name: "John", age: 30 };

// Assign using the custom method
const customInstance = new CustomType();
customInstance.assignFromObject(plainObject);

console.log(customInstance instanceof CustomType); // true
console.log(customInstance); // CustomType { name: 'John', age: 30 }

Scenario 4: Using Static Methods

A static method can return a new instance based on a plain object:

class CustomType {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }

  static fromObject(obj) {
    return new CustomType(obj.name, obj.age);
  }
}

// Plain object
const plainObject = { name: "John", age: 30 };

// Create instance using static method
const customInstance = CustomType.fromObject(plainObject);

console.log(customInstance instanceof CustomType); // true
console.log(customInstance); // CustomType { name: 'John', age: 30 }

Key Considerations

  • Validation: Ensure the source object has the expected properties and types.
  • Prototypes: Direct assignment (Object.assign) won't copy methods or prototype chain relationships.
  • Deep Copy: Use libraries like lodash if the object has nested structures that need cloning.

Choose the approach based on your specific needs for validation, transformation, and maintainability.

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