Question 228818 in Javascript Question posted in
A very good W3school tutorial can be found here.

Javascript – Creating new instance of the same class as an existing instance, without using eval()

I have an instance sampleInstance of an unknown class and I need to create a second instance of the same class. Here’s what I am doing right now:

  1. I look up the class name with sampleInstance.constructor.name.
  2. Next, I use eval(`new ${sampleInstance.constructor.name}()`) to create a new instance of the same class.

The code below works fine:

class sampleClass_A { name = "Archimedes";}
class sampleClass_B { name = "Pythagoras";}

let sampleInstance = new sampleClass_A();

// later, in another part of the code, I don't know the constructor of sampleInstance anymore
let constructorName = sampleInstance.constructor.name
let newInstanceOfTheSameClass = eval(`new ${constructorName}()`); // how to do without eval()???
console.log(newInstanceOfTheSameClass .name); // "Archimedes"

But I’d rather not use eval(). What’s a cleaner alternative?

(I cannot use window[constructorName] because this code will not run in a browser in the general case. (Not even sure if it would work in a browser.))

Tags:

3

Answers


  1. 0

    Call the constructor directly !

    const aCopy = new (sampleInstance.constructor as any)();
console.log(aCopy);

    The only downside I see is that the constructor is not type, so you’ll have to cast it and the returned value won’t be typed.

    Playground

    Login or Signup to reply.
  2. 0

    Assuming you’re using typescript, as seen in this instance code

    class sampleClass_A { name = "Archimedes";}

    I created a fiddle for you:

    class sampleClass_A { name = "Archimedes";}
class sampleClass_B { name = "Pythagoras";}

let sampleInstance = new sampleClass_A();

// later, in another part of the code, I don't know the constructor of sampleInstance anymore
//let constructorName = sampleInstance.constructor.name;
//let newInstanceOfTheSameClass = eval(`new ${constructorName}()`); // how to do without eval()???

let newInstanceOfTheSameClass = new sampleInstance.constructor();
console.log(newInstanceOfTheSameClass .name); // "Archimedes"

    https://jsfiddle.net/t9gdasur/

    Let me know if it works. if not, I’ll try something else.

    Login or Signup to reply.
  3. 0

    1- Get a reference to the constructor function of the existing
    instance using its prototype.

    2- Use the retrieved constructor to create a new instance of the same
    class.

    class sampleClass_A { name = "Archimedes"; }
class sampleClass_B { name = "Pythagoras"; }

let sampleInstance = new sampleClass_A();

// Step 1: Retrieve the constructor
const constructor = sampleInstance.__proto__.constructor;

// Step 2: Create a new instance
let newInstanceOfTheSameClass = new constructor();

console.log(newInstanceOfTheSameClass.name); // "Archimedes"
    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search