Casting Generic Objects to Typed Instances in JavaScript
Problem Statement:
JSON responses from servers often contain plain objects that represent class instances. Casting these generic objects to typed instances allows for utilizing their class methods and accessing typed data.
Solution:
Creating object instances requires invoking their constructors. However, JSON data lacks constructor information, so a different approach is necessary.
1. Iterative Cloning:
This approach involves cloning the properties of the plain objects to newly created instances:
<code class="javascript">var personLiteral = ...; // JSON.parse("...");
var personInstance = new Person();
for (var prop in personLiteral)
personInstance[prop] = personLiteral[prop];</code>2. Using Object.assign:
Object.assign can be used to accomplish the same task more concisely:
<code class="javascript">var personInstance = Object.assign(new Person(), personLiteral);</code>
For more complex objects with nested structures, iterate through the properties and apply the same cloning process recursively.
3. Custom Constructor Cloning:
Every constructor can be extended to accept plain objects and clone them, handling any necessary logic:
<code class="javascript">Person.fromJSON = function(obj) {
// custom code, as appropriate for Person instances
// might invoke `new Person`
return …;
};</code>Example:
In your specific scenario, apply the cloning process to both the Person and Animal objects:
<code class="javascript">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]);
}
}</code>Remember that run methods should be added to the Animal prototype rather than individual instances.
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