Why do I get an undefined value from the defineProperty method?

Question

var funcSetter = {

    defineProperty: function(target, prop, descriptor) {
        if (prop) {
            let temp = descriptor.value;
            descriptor.value = temp => {
                if (temp.startsWith('_')) {
                    temp = "Default Value Attached , no Underscores allowed";
                    return temp;
                } else return temp;
            };
        }
        return true;
    }
};

let proxy_3 = new Proxy(obj_3, funcSetter);

Object.defineProperty(proxy_3, 'no', {
    value: '_Bharath',
    writable: true,
    enumerable: true,
    configurable: true
});

The issue I am facing here is that when I call the trap defineProperty, the arrow function defined under descriptor.value does not get called, hits the return true at the bottom and sets the value of the property as undefined

I am quite sure I haven't used the arrow function correctly. Could anyone guide me in the right direction?

Thank you for all the tips. Much appreciated!

Answer 1

There are two problems:

1. You're just assigning the arrow function to descriptor.value. Not calling it.
2. You're not actually setting the property anywhere.

I think this should solve the problem

var funcSetter = {

  defineProperty: function(target, prop, descriptor) {
    if (prop) {
      let temp = descriptor.value;
      // Use an IIFE
      descriptor.value = (temp => {
        if (temp.startsWith('_')) {
          return "Default Value Attached , no Underscores allowed";
        } else {
          return temp;
        };
      })(temp);
    }
    // Use Reflect.defineProperty to actually set the property
    return Reflect.defineProperty(target, prop, descriptor);
  }
};

let obj_3 = {};
let proxy_3 = new Proxy(obj_3, funcSetter);

Object.defineProperty(proxy_3, 'no', {
  value: '_Bharath',
  writable: true,
  enumerable: true,
  configurable: true
});

console.log(obj_3);

Also, a MUCH simpler way to do this

var funcSetter = {

  defineProperty: function(target, prop, descriptor) {
    if (prop) {
      if (descriptor.value.startsWith('_')) {
        descriptor.value = "Default Value Attached , no Underscores allowed";
      }
    }
    // Use Reflect.defineProperty to actually set the property
    return Reflect.defineProperty(target, prop, descriptor);
  }
}
let obj_3 = {};
let proxy_3 = new Proxy(obj_3, funcSetter);

Object.defineProperty(proxy_3, 'no', {
  value: '_Bharath',
  writable: true,
  enumerable: true,
  configurable: true
});

console.log(obj_3);