Constructor Functions

What they are: Special functions that create and set up new objects when called with new

Constructor Function Rules:

1. Naming convention: Start with capital letter

javascript

function User(name) {     // ✅ Capital letter
  this.name = name;
}
 
function createUser(name) { // ❌ Not a constructor naming
  this.name = name;
}

2. Must be called with new keyword

javascript

function User(name) {
  this.name = name;
  this.isAdmin = false;
}
 
const user1 = new User("John");    // ✅ Correct
const user2 = User("Jane");        // ❌ Wrong (this = undefined)

What new Does Behind the Scenes:

javascript

function User(name) {
  // Step 1: this = {} (new empty object created)
  
  // Step 2: Add properties to this
  this.name = name;
  this.isAdmin = false;
  
  // Step 3: return this (automatic)
}
 
// These are equivalent:
const user = new User("John");
 
const user = {
  name: "John", 
  isAdmin: false
};

Constructor vs Regular Function:

javascript

function User(name) {
  this.name = name;
}
 
// Called as constructor
const user1 = new User("John");
console.log(user1.name); // "John"
 
// Called as regular function
const user2 = User("Jane"); 
console.log(user2); // undefined (function returns nothing)
console.log(window.name); // "Jane" (this = window in non-strict)

Return Statement Rules:

1. Return object → returns that object

javascript

function User(name) {
  this.name = name;
  return { name: "Override" }; // Returns this instead of this
}
 
const user = new User("John");
console.log(user.name); // "Override"

2. Return primitive → ignored, returns this

javascript

function User(name) {
  this.name = name;
  return "ignored"; // Primitive ignored
}
 
const user = new User("John");
console.log(user.name); // "John" (returns this)

Adding Methods to Constructor:

javascript

function User(name, age) {
  this.name = name;
  this.age = age;
  
  this.sayHi = function() {
    console.log(`Hi, I'm ${this.name}`);
  };
  
  this.getInfo = function() {
    return `${this.name} is ${this.age} years old`;
  };
}
 
const user = new User("John", 30);
user.sayHi(); // "Hi, I'm John"
console.log(user.getInfo()); // "John is 30 years old"

---

Interview Gotchas:

javascript

// Gotcha 1: Forgetting 'new'
function User(name) { this.name = name; }
const user = User("John"); // undefined, window.name = "John"
 
// Gotcha 2: Arrow functions can't be constructors
const User = (name) => { this.name = name; };
const user = new User("John"); // TypeError!
 
// Gotcha 3: Return object overrides
function User(name) {
  this.name = name;
  return {}; // Returns empty object, not {name: "John"}
}
 
// Gotcha 4: Each instance gets own method copy
function User(name) {
  this.sayHi = function() {}; // New function for each instance!
}
// Better: Use prototypes for shared methods

Constructor vs Class (Preview):

javascript

// Constructor function
function User(name) {
  this.name = name;
}
 
// ES6 Class (same thing, different syntax)
class User {
  constructor(name) {
    this.name = name;
  }
}

Bottom line: Constructor functions are factories for creating similar objects. Use capital letters, call with new, and this becomes the new object being created!