New

The new operator creates an instance of a user-defined object type or of one of the built-in object types that possesses a constructor function. It fundamentally alters the execution context of a function call, shifting it from a standard invocation to a constructor invocation.

new constructor
new constructor()
new constructor(arg1, arg2, /* …, */ argN)

When a function is invoked with the new keyword, the JavaScript engine executes a strict four-step internal sequence:

1. Object Creation: A new, blank JavaScript object is allocated in memory.
2. Prototype Linkage: The newly created object’s internal [[Prototype]] is bound to the constructor function’s prototype property. If the constructor’s prototype is a primitive value (including null), the engine safely falls back to binding the [[Prototype]] to Object.prototype.
3. Context Binding: The constructor function is executed with the newly created object bound to the this execution context. Any properties or methods added to this within the constructor are attached directly to the new object.
4. Implicit Return: The engine evaluates the constructor’s return value. If the constructor explicitly returns a non-primitive object (e.g., an Object, Array, or Function), that object is returned, and the newly created object from Step 1 is discarded. If the constructor returns a primitive value (or returns nothing), the engine implicitly returns the newly created object referenced by this.

To visualize these internal mechanics, the behavior of the new operator can be represented by the following equivalent function:

function simulateNew(constructor, ...args) {
  // Step 1 & 2: Create a new object and link its [[Prototype]]
  // Fall back to Object.prototype if constructor.prototype is a primitive or null
  let proto = constructor.prototype;
  if ((typeof proto !== 'object' && typeof proto !== 'function') || proto === null) {
    proto = Object.prototype;
  }
  const instance = Object.create(proto);
  
  // Step 3: Execute the constructor with 'this' bound to the new instance
  const result = constructor.apply(instance, args);
  
  // Step 4: Return the explicit object result, or the generated instance
  const isObject = typeof result === 'object' && result !== null;
  const isFunction = typeof result === 'function';
  
  return (isObject || isFunction) ? result : instance;
}

Constructor Constraints

Not all functions can be invoked with the new operator. The engine enforces specific constraints based on the function type:

• Arrow Functions: Arrow functions cannot be used with the new operator. They lack the [[Construct]] internal method and do not possess a prototype property. Attempting to instantiate an arrow function will throw a TypeError.
• ES6 Classes: ES6 class constructors strictly require the new keyword. Unlike standard functions, invoking a class constructor as a standard function (without new) will throw a TypeError.

The new.target Meta-Property

When a function is executed, the engine populates a pseudo-property named new.target. This allows developers to detect at runtime whether a function was invoked via the new operator.

• If the function was called with new, new.target holds a reference to the constructor function or class that was initially invoked. In the context of class inheritance, if a parent class is instantiated via a subclass (e.g., via super()), new.target within the parent constructor refers to the subclass that was initially invoked, not the parent constructor.
• If the function was called via standard invocation, new.target evaluates to undefined.

function ConstructorCheck() {
  if (!new.target) {
    throw new TypeError("ConstructorCheck must be called with the 'new' operator.");
  }
  // Initialization logic proceeds here
}

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