Get arguments of constructor in JS class

Question

How can i get an array of the arguments of a constructor class in JS? It's possible? Thanks in advance.

class Product {
    constructor(id, name, price, category, stock){
      this.id = id;
      this.name = name;
      this.price = price;
      this.category = category;
      this.stock = stock;
    }  
};

console.log(Product.constructor.params);

//expected output = ['id', 'name', 'price', 'category', 'stock'];

Without using type notation or a typed language like typescript? — evolutionxbox, Jul 10 '21 at 22:04
Does this help? https://stackoverflow.com/questions/3404057/determine-original-name-of-variable-after-its-passed-to-a-function — evolutionxbox, Jul 10 '21 at 22:07
What are you trying to do? You already know the arity/arguments of your constructor, and the class properties. — Andy, Jul 10 '21 at 22:09
I need to get the constructor arguments to compare and filter with another logic in my code — sonEtLumiere, Jul 10 '21 at 22:17
My point is that you already _know_ what the class properties are. You specified them. Are they going to change? — Andy, Jul 10 '21 at 22:26
No, will not change. But if i add some new property in the constructor i have to update several pieces of code validation. The instances of Products sometimes are converted to strings so for that reason i would like to validate with no need to make an instance of a new product again. — sonEtLumiere, Jul 10 '21 at 22:35

Louys Patrice Bessette · Answer 1 · 2021-07-10T22:58:40.850

3

Inspired form @Porter answer and @evolutionxbox, I think that a reliable way would be using .match() like this:

class Product {
  constructor(id, name, price, category, stock, unusedArgument) {
    this.id = id;
    this.name = name;
    this.price = price;
    this.category = category;
    this.stock = stock;
  }
}

class Noarguments {
  constructor() {
  }
}

// A function
function getClassContructorParams(obj){
  let match = obj.toString().match(/constructor\((.+)\)/)
  
  if(match && match[1]){
    return match[1].split(",");
  }
  
  // If no match
  return []  
}


console.log(getClassContructorParams(Product))

/// Testing a class with no constructor arguments will return an empty array
console.log(getClassContructorParams(Noarguments))

My previous answer was returning the object properties, which may be different from the arguments used in the constructor...

Now using .match() will ensure what is returned really are the constructor arguments.

edited Jul 10 '21 at 22:58

answered Jul 10 '21 at 22:07

Louys Patrice Bessette

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1

You don't need dummy arguments at all. `Object.keys(new Product)` produces the same result. – Unmitigated Jul 10 '21 at 22:09
2

this only works if every argument has a one-to-one mapping to a property. What if any of the names of the arguments are different to the properties? – evolutionxbox Jul 10 '21 at 22:10
That works, i just would like to access the constructor arguments with no need to instance a new Product – sonEtLumiere Jul 10 '21 at 22:11
@sonEtLumiere: I edited with something more accurate. – Louys Patrice Bessette Jul 10 '21 at 22:59

score 2 · Answer 2 · answered Jul 10 '21 at 22:08

2

let s = Product.toString();
let params = s.substring(s.indexOf('(')+1, s.indexOf(')')).split(',')

answered Jul 10 '21 at 22:08

Porter

193
1
8

Andy · Answer 3 · 2021-07-11T11:11:10.580

My point in the comments was it seems you're coming at this problem from the wrong direction. Maybe take this approach. Have an array of labels, and array of data, and pass those into the class. You'll still have the array of labels to access (and validate) in your other code, and everything will still work.

const labels = [ 'id', 'name', 'price', 'category', 'stock' ];

const data = [ 1, 'Bob', 100, 2, 1];

class Product {

    constructor(labels, data) {
      data.forEach((el, i) => this[labels[i]] = el);
    }
    
};

console.log(new Product(labels, data));
console.log(labels);

Or, if your products are identical in terms of properties you could just use an array of them, and use Object.keys to get the labels of the first object.

const data = [{ id: 1, name: 'Bob', price: 100, category: 2, stock: 1 }];
const labels = Object.keys(data[0]);

class Product {

    constructor(data) {
      for (let key in data) {
        this[key] = data[key];
      }
    }
    
};

console.log(new Product(data[0]));
console.log(labels);

Get arguments of constructor in JS class

3 Answers3

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