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Objects in JavaScript

Objects are key/value maps and the fundamental building block for structured data.

Creating objects

const o1 = { a: 1, b: 2 }
const o2 = Object.create(null) // no prototype

Property access

Dot vs bracket notation; computed keys.

o1.a
const k = 'b'
o1[k]
const key = `x_${Date.now()}`
const obj = { [key]: 42 }

Shorthand and methods

const a = 1
const user = {
  a,                  // shorthand property
  greet(name) { return `Hi ${name}` }, // method shorthand
}

Descriptors and immutability

Control writability, enumerability, and configurability.

Object.defineProperty(o1, 'hidden', { value: 1, writable: false })
Object.freeze(o1)        // shallow immutable
Object.seal(o1)          // no add/remove

Prototype and __proto__

Every object (except Object.create(null)) has a prototype. Prefer Object.getPrototypeOf/setPrototypeOf.

const p = { z: 9 }
const c = Object.create(p)

Copy and merge

const copy = { ...o1 }
const merged = { ...o1, ...{ c: 3 } }
Object.assign({}, o1, { c: 3 })

JSON

Serialize/deserialize data structures.

const txt = JSON.stringify({ a: 1 })
const back = JSON.parse(txt)

hasOwn and iteration

Object.hasOwn(o1, 'a')     // ES2022
for (const k of Object.keys(o1)) {}
for (const [k,v] of Object.entries(o1)) {}

Summary

  • Use literals and shorthand; compute keys with brackets
  • Control mutability with descriptors/freeze; use spread/assign to copy/merge
  • Prefer Object.hasOwn/Object.keys for safe iteration
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