How to Cast Objects to Class Instances in JavaScript

Casting Objects to Class Instances in JavaScript

Many cases arise when developers encounter the need to cast plain JavaScript objects, typically received from a server response, into specific class instances for further processing. This requirement arises when the objects received from the server lack the necessary methods and properties associated with the class they represent.

Prerequisites

To cast objects into class instances, we must first understand the following:

• In JavaScript, creating an object instances requires invoking its constructor.
• The constructor takes specific arguments, which may extend beyond simple properties.
• After invoking the constructor, we must assign public properties from the parsed JSON object to the created instances.

Solution

A robust solution is to ensure that every constructor can accept objects resembling instances (including actual instances) and clone them appropriately. This ensures that the instance creation logic is correctly handled.

An alternative and more efficient approach is to create a static method within the class to convert objects into instances:

Person.fromJSON = function(obj) {
    // Custom code for creating instances of Person
    return …;
};

Specific Example

Consider the following simplified example:

function Person() {
    this.personName = "";
    this.animals = [];
}

function Animal(){
    this.animalName = "";
    this.run = function(meters){
        .....
    }
}

To convert the JSON response, we can use the following approach:

var persons = JSON.parse(serverResponse);
for (var i=0; i<persons.length; i++) {
    persons[i] = $.extend(new Person, persons[i]);
    for (var j=0; j<persons[i].animals; j++) {
        persons[i].animals[j] = $.extend(new Animal, persons[i].animals[j]);
    }
}

Note: The provided run method should be added to the Animal.prototype object rather than individual instances.

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