new Operator

JavaScript · Reference cheat sheet

📋 Overview

new constructs an instance from a constructor function or class. It creates an object, sets its prototype, binds this, and returns the object (unless the constructor returns another object).

🔧 Core concepts

• Steps: create \{\} → set [[Prototype]] → call constructor with this → return object (or explicit object return).
• Classes: new Person() required; calling without new throws.
• new.target: meta-property detecting construct calls.
• new.target / subclassing: works with extends and super.
• Built-ins: new Map(), new Date(), new Error() — some built-ins differ if called as functions.

class Point {
  constructor(x, y) {
    this.x = x;
    this.y = y;
  }
}
const p = new Point(1, 2);

💡 Examples

// Classic constructor
function User(name) {
  if (new.target === undefined) {
    return new User(name); // allow factory-style call
  }
  this.name = name;
}
User.prototype.hello = function () {
  return `Hi, ${this.name}`;
};

const u = new User("Ada");
u instanceof User; // true
Object.getPrototypeOf(u) === User.prototype; // true

// Class fields & private
class Counter {
  #n = 0;
  get value() {
    return this.#n;
  }
  inc() {
    this.#n += 1;
  }
}
const c = new Counter();

// Return override
function Weird() {
  this.a = 1;
  return { b: 2 }; // instance discarded
}
new Weird(); // { b: 2 }

// Reflect.construct
const d = Reflect.construct(Date, [2026, 0, 1]);

// Array exotic
const a = new Array(3); // length 3 sparse
const b = Array.of(3); // [3]

⚠️ Pitfalls

• Forgetting new on old constructors sets properties on the global/undefined (strict) — prefer class.
• Returning a primitive from a constructor is ignored; returning an object replaces this.
• arrow functions cannot be constructors — no new.
• Symbol / BigInt are not constructible with new.
• instanceof can be spoofed; use brands or new.target checks for security-sensitive code.

🔗 Related

• oop.md — classes & inheritance
• objects.md — prototypes
• properties.md — defining instance props
• error.md — new Error
• map.md — new Map
• date.md — new Date