Properties

JavaScript · Reference cheat sheet

📋 Overview

Object properties have descriptors controlling writability, enumerability, configurability, and accessors. Understanding descriptors explains getters, Object.defineProperty, and class fields.

🔧 Core concepts

Data descriptor: value, writable, enumerable, configurable.
Accessor descriptor: get, set, enumerable, configurable.
Define: Object.defineProperty, Object.defineProperties.
Inspect: Object.getOwnPropertyDescriptor, Object.getOwnPropertyDescriptors.
Flags: non-writable / non-configurable lock down APIs.
Symbols: can be keys; often non-enumerable by convention.

Object.defineProperty(obj, "id", {
  value: 1,
  writable: false,
  enumerable: true,
  configurable: false,
});

💡 Examples

const user = {};

Object.defineProperty(user, "name", {
  value: "Ada",
  writable: true,
  enumerable: true,
  configurable: true,
});

// Accessor
Object.defineProperty(user, "tag", {
  get() {
    return `@${this.name.toLowerCase()}`;
  },
  set(v) {
    this.name = String(v).replace(/^@/, "");
  },
  enumerable: true,
});

user.tag = "@Grace";
console.log(user.name); // Grace

// Multiple
Object.defineProperties(user, {
  role: { value: "admin", enumerable: true },
  createdAt: { value: Date.now(), writable: false },
});

// Class equivalent
class Rect {
  #w;
  constructor(w, h) {
    this.#w = w;
    this.height = h;
  }
  get width() {
    return this.#w;
  }
  get area() {
    return this.#w * this.height;
  }
}

// Copy descriptors (not just values)
const clone = Object.defineProperties(
  {},
  Object.getOwnPropertyDescriptors(user),
);

// Prevent extensions
Object.preventExtensions(user);
Object.seal(user); // + non-configurable
Object.freeze(user); // + non-writable data props

⚠️ Pitfalls

Assignment silently fails on non-writable props in non-strict mode; throws in strict.
You cannot mix value and get/set in one descriptor.
configurable: false prevents deleting or redefining later.
Spread/Object.assign copy enumerable values, not getters as accessors.
Class fields are writable/enumerable on the instance by default (not on prototype).

objects.md — object basics
oop.md — class getters/fields
new.md — instance creation
map.md — alternative key storage
json.md — enumerable own props only

Properties

Properties

📋 Overview

🔧 Core concepts

💡 Examples

⚠️ Pitfalls

🔗 Related

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