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Javascript – How to detect if an object property is read-only (doesn't have a setter)?

Having an object that is made from a class definition:

class Something {
   get prop() {
      ...
   }
}

const s = new Something();

How to detect in code that s.prop is read-only property and cannot be set (and without trying to set it)? What I’ve already tried is to use Reflect.getOwnPropertyDescriptor but it returns undefined in this case.

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2

Answers


  1. 0

    Stolen and adapted from https://stackoverflow.com/a/70250484/13111630, but you can use these functions to find all the setters and getters for your class.

    function listGetters (instance) {
  return Object.entries(
    Object.getOwnPropertyDescriptors(
      Reflect.getPrototypeOf(instance)
    )
  )
  .filter(e => typeof e[1].get === 'function' && e[0] !== '__proto__')
  .map(e => e[0]);
}

function listSetters(instance) {
    return Object.entries(
    Object.getOwnPropertyDescriptors(
      Reflect.getPrototypeOf(instance)
    )
  )
  .filter(e => typeof e[1].set === 'function' && e[0] !== '__proto__')
  .map(e => e[0]);
}

    In your example, you can run listSetters on your Something instance to see that there’s a prop getter but no prop setter.

    class Something {
  get prop() {
    return "foo"
  }
}

const s = new Something();


function listGetters(instance) {
  return Object.entries(
      Object.getOwnPropertyDescriptors(
        Reflect.getPrototypeOf(instance)
      )
    )
    .filter(e => typeof e[1].get === 'function' && e[0] !== '__proto__')
    .map(e => e[0]);
}

function listSetters(instance) {
  return Object.entries(
      Object.getOwnPropertyDescriptors(
        Reflect.getPrototypeOf(instance)
      )
    )
    .filter(e => typeof e[1].set === 'function' && e[0] !== '__proto__')
    .map(e => e[0]);
}

console.log(listGetters(s));
console.log(listSetters(s));
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  2. 0

    Try this:

    function isPropertyWritable(object, property) {
    if (!(property in object))
        throw new Error('Property not in object');

    let descriptor = Reflect.getOwnPropertyDescriptor(object, property);
    if (!descriptor) {
        let prototype = Reflect.getPrototypeOf(object);
        while (prototype) {
            descriptor = Reflect.getOwnPropertyDescriptor(prototype, property);
            if (descriptor)
                break;
            prototype = Reflect.getPrototypeOf(prototype);
        }
    }

    return 'writable' in descriptor ? descriptor.writable : descriptor.set != null;
}
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