Detailed explanation of JavaScript Proxy object

// Syntax const p = new Proxy(target, handler)

// 1. Find a suitable handler and write code const handler = {
    get: function(obj, prop) {
        return prop in obj ? obj[prop] : 37;
    }
};
// 2. Create a new Proxy object const p = new Proxy({}, handler);
// 3. Execute operation pa = 1;
pb = undefined;
// 4. Check the results console.log(pa, pb); // 1, undefined
console.log('c' in p, pc); // false, 37

let target = {};
let p = new Proxy(target, {});
pa = 37; // The operation is forwarded to the target console.log(target.a); // 37. The operation has been forwarded correctly

let validator = {
  set: function(obj, prop, value) {
    if (prop === 'age') {
      if (!Number.isInteger(value)) {
        throw new TypeError('The age is not an integer');
      }
      if (value > 200) {
        throw new RangeError('The age seems invalid');
      }
    }
    // The default behavior to store the value
    obj[prop] = value;
    // Indicates success return true;
  }
};
let person = new Proxy({}, validator);
person.age = 100;
console.log(person.age);
// 100
person.age = 'young';
// Throws an exception: Uncaught TypeError: The age is not an integer
person.age = 300;
// Throws an exception: Uncaught RangeError: The age seems invalid

function extend(sup, base) {
  var descriptor = Object.getOwnPropertyDescriptor(
    base.prototype, "constructor"
  );
  base.prototype = Object.create(sup.prototype);
  var handler = {
    construct: function(target, args) {
      var obj = Object.create(base.prototype);
      this.apply(target, obj, args);
      return obj;
    },
    apply: function(target, that, args) {
      sup.apply(that, args);
      base.apply(that, args);
    }
  };
  var proxy = new Proxy(base, handler);
  descriptor.value = proxy;
  Object.defineProperty(base.prototype, "constructor", descriptor);
  return proxy;
}
var Person = function (name) {
  this.name = name
};
var Boy = extend(Person, function (name, age) {
  this.age = age;
});
Boy.prototype.sex = "M";
var Peter = new Boy("Peter", 13);
console.log(Peter.sex); // "M"
console.log(Peter.name); // "Peter"
console.log(Peter.age); // 13