Is it possible to retrieve a class's array of properties without creating an instance of it?

Question

Is it possible to retrieve a class's array of properties without creating an instance of it?

It appears that there might be a way to accomplish this task by dynamically constructing a form based on a class definition (Angular) without being dependent on the specific class itself. This approach would allow for scalability, enabling the addition of fields to the class without needing to update the form logic and template.

Is there a method or possibly an NPM package available to achieve this?

I have come across using ClassName.toString(), but parsing it could prove difficult. If necessary, I may consider developing a module to handle this task.

The idea of creating a dummy instance of the class solely for property enumeration purposes seems like a less efficient route to take.

javascript npm

Answer 1

Answer №1

If you're looking to retrieve all the properties of an object in JavaScript, one way to do so is by using the method Object.getOwnPropertyNames().

Consider the following example class:

class Foo {
  setBar() {
    throw Error('not implemented');
    return false;
  }
  getBar() {
    throw Error('not implemented');
    return false;
  }
}

To get a list of the properties defined on the prototype of this class, you can use:

Object.getOwnPropertyNames(Foo.prototype)

This will result in an array containing:

["constructor", "setBar", "getBar"]

In my research, I initially explored using Object.keys() as an alternative. While it didn't provide the desired outcome, you may find the documentation for the polyfill for Object.keys() helpful. It includes code for filtering out certain default properties like constructor and toString, and also correctly implements hasOwnProperty.

For more insights on listing down all prototype properties of a JavaScript object, you can refer to Bergi's answer on this thread.

Answer 2

If you're looking to retrieve all the properties of an object in JavaScript, one way to do so is by using the method Object.getOwnPropertyNames().

Consider the following example class:

class Foo {
  setBar() {
    throw Error('not implemented');
    return false;
  }
  getBar() {
    throw Error('not implemented');
    return false;
  }
}

To get a list of the properties defined on the prototype of this class, you can use:

Object.getOwnPropertyNames(Foo.prototype)

This will result in an array containing:

["constructor", "setBar", "getBar"]

In my research, I initially explored using Object.keys() as an alternative. While it didn't provide the desired outcome, you may find the documentation for the polyfill for Object.keys() helpful. It includes code for filtering out certain default properties like constructor and toString, and also correctly implements hasOwnProperty.

For more insights on listing down all prototype properties of a JavaScript object, you can refer to Bergi's answer on this thread.

Answer 3

Answer №2

Is there another approach? You can define your class as a function and assign properties to its prototype:

var Monarchs = function() {
};

Monarchs.prototype = {
  "henry" : "viii",
  "elizabeth" : "i"
}

console.log(Object.keys(Monarchs.prototype))

// Output will be
// [ 'henry', 'elizabeth' ]

Answer 4

Is there another approach? You can define your class as a function and assign properties to its prototype:

var Monarchs = function() {
};

Monarchs.prototype = {
  "henry" : "viii",
  "elizabeth" : "i"
}

console.log(Object.keys(Monarchs.prototype))

// Output will be
// [ 'henry', 'elizabeth' ]

Answer 5

Answer №3

Accessing the properties of a class prototype without instantiation may result in only methods and accessor descriptors being exposed - data properties remain undisclosed until actual instantiation due to the potential influence constructor arguments can have on the properties' types, values, and quantities. Instances may not be desired for various reasons, such as tracking static counters; hence, a possible solution is to create a "shadow" duplicate of the class dynamically and instantiate it with sample constructor arguments.</p>
<p><div>
<div>
<pre class="lang-js"><code>    /**
     * Obtain properties from a class definition without creating instances
     *
     * @param cls: class
     * @param args: example arguments to provide to the shadow constructor
     * @usage `const props = getInstanceProperties(MyClass);`
     * @notice this will regex replace the classname (potential issue with strings containing that substring)
     */
    const getInstanceProperties = (cls, args = [], ignore = ['constructor', 'toString']) => {
        const className = cls.prototype.constructor.name;
        const shadowCode = cls.toString().replace(new RegExp(`${className}`, 'g'), `_${className}_`);
        const shadowClass = eval(`(${shadowCode})`);
        const o = new shadowClass(...args);
        const methodsAndAccessors = Object.getOwnPropertyDescriptors(cls.prototype);
        const dataDescriptors = Object.getOwnPropertyDescriptors(o);
        const descriptors = Object.assign({}, methodsAndAccessors, dataDescriptors);
        ignore.forEach(name => delete descriptors[name]);
        return descriptors;
    };


    class Foo extends Object {
      static instances = 0;
      #myPrivateVar = 123;
      myValue=123;
      constructor(){
          super();
          this.myConstructorProp = ++Foo.instances;
      }
      myMethod() {}
      set myAccessor(x){}
    }

    console.log(Object.keys(getInstanceProperties(Foo)));

Answer 6

Accessing the properties of a class prototype without instantiation may result in only methods and accessor descriptors being exposed - data properties remain undisclosed until actual instantiation due to the potential influence constructor arguments can have on the properties' types, values, and quantities. Instances may not be desired for various reasons, such as tracking static counters; hence, a possible solution is to create a "shadow" duplicate of the class dynamically and instantiate it with sample constructor arguments.</p>
<p><div>
<div>
<pre class="lang-js"><code>    /**
     * Obtain properties from a class definition without creating instances
     *
     * @param cls: class
     * @param args: example arguments to provide to the shadow constructor
     * @usage `const props = getInstanceProperties(MyClass);`
     * @notice this will regex replace the classname (potential issue with strings containing that substring)
     */
    const getInstanceProperties = (cls, args = [], ignore = ['constructor', 'toString']) => {
        const className = cls.prototype.constructor.name;
        const shadowCode = cls.toString().replace(new RegExp(`${className}`, 'g'), `_${className}_`);
        const shadowClass = eval(`(${shadowCode})`);
        const o = new shadowClass(...args);
        const methodsAndAccessors = Object.getOwnPropertyDescriptors(cls.prototype);
        const dataDescriptors = Object.getOwnPropertyDescriptors(o);
        const descriptors = Object.assign({}, methodsAndAccessors, dataDescriptors);
        ignore.forEach(name => delete descriptors[name]);
        return descriptors;
    };


    class Foo extends Object {
      static instances = 0;
      #myPrivateVar = 123;
      myValue=123;
      constructor(){
          super();
          this.myConstructorProp = ++Foo.instances;
      }
      myMethod() {}
      set myAccessor(x){}
    }

    console.log(Object.keys(getInstanceProperties(Foo)));

Is it possible to retrieve a class's array of properties without creating an instance of it?

Answer №1

Answer №2

Answer №3

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