How the new Operator Works

When using the new operator, the function that follows it executes the following steps in order:

1. Create an empty object that will serve as the instance to be returned.
2. Set the prototype of this empty object to point to the constructor's prototype property.
3. Assign this empty object to the this keyword inside the function.
4. Execute the code inside the constructor.
5. If the constructor has a return value and it is an object type, return that object; otherwise, return the instance object created above.

// Constructor
function Person(name,age){
    this.name = name
    this.age = age
}

// Custom _new
function _new() {
  // Convert the arguments object to an array
  var args = [].slice.call(arguments);
  // Extract the constructor
  var constructor = args.shift();
  // Create an empty object that inherits the constructor's prototype property
  var context = Object.create(constructor.prototype);
  // Execute the constructor, assigning the context object to the internal this
  var result = constructor.apply(context, args);
  // If the result is an object, return it directly; otherwise, return the context object
  return (typeof result === 'object' && result != null) ? result : context;
}

// Custom _new2
function _new2(/* constructor */ constructor, /* constructor params */ params) {
  // Create an empty object that inherits the constructor's prototype property
  var context = Object.create(constructor.prototype);
  // Execute the constructor, assigning the context object to the internal this
  var result = constructor.apply(context, params);
  // If the result is an object, return it directly; otherwise, return the context object
  return (typeof result === 'object' && result != null) ? result : context;
  // (If the user defines a custom return object inside the constructor, use that object; otherwise, return context)
}


// Create an instance via custom _new
var actor = _new(Person, 'Zhang San', 28);
console.log(actor.name) // Zhang San

// Create an instance via custom _new2
var actor2 = _new2(Person, ['Li Si', 29]);
console.log(actor2.name) // Li Si

// Create an instance via the new operator
var actor3 = new Person('Wang Wu',30)
console.log(actor3.name) // Wang Wu