JavaScript Academy · Lesson

Symbols as Object Keys

Use symbols for non-colliding properties.

Lesson 2 of 413 steps

Symbols as Object Keys is a free JavaScript Academy lesson on CoddyKit. This is lesson 2 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the JavaScript Academy learning path, and your progress syncs across the web and the CoddyKit app. The JavaScript Academy course includes 4 lessons in total.

Symbols as Keys

A symbol can be used as an object property key. Because each symbol is unique, such a property cannot accidentally collide with any string key.

const id = Symbol("id")
const user = {}
user[id] = 123
console.log(user[id])

Computed Symbol Keys

To use a symbol as a key in an object literal, wrap it in square brackets as a computed property name.

const id = Symbol("id")
const user = {
  name: "Ann",
  [id]: 99
}
console.log(user[id])

Reading a Symbol Property

You must hold a reference to the same symbol to read its property. Without that reference, the value is effectively hidden.

const secret = Symbol("secret")
const obj = { [secret]: "hidden" }
console.log(obj[secret])

Not Enumerable in for-in

Symbol keys are skipped by a for-in loop. This keeps symbol properties out of ordinary iteration.

const tag = Symbol("tag")
const obj = { a: 1, b: 2, [tag]: 3 }
for (const key in obj) {
  console.log(key)
}

Skipped by Object.keys

Object.keys returns only string keys, so symbol-keyed properties are excluded from it as well.

const tag = Symbol("tag")
const obj = { a: 1, [tag]: 2 }
console.log(Object.keys(obj))

Hidden From JSON

JSON.stringify ignores symbol keys entirely, so they never appear in serialized output. This makes them useful for internal metadata.

const meta = Symbol("meta")
const obj = { name: "Bo", [meta]: "internal" }
console.log(JSON.stringify(obj))

Finding Symbol Keys

To list symbol keys, use Object.getOwnPropertySymbols, which returns an array of the symbol properties on an object.

const tag = Symbol("tag")
const obj = { a: 1, [tag]: 2 }
const syms = Object.getOwnPropertySymbols(obj)
console.log(syms.length)

Avoiding Name Clashes

Symbol keys let different libraries add properties to the same object without risk of overwriting each other, since their symbols differ.

const libA = Symbol("data")
const libB = Symbol("data")
const shared = {}
shared[libA] = 1
shared[libB] = 2
console.log(shared[libA], shared[libB])

Symbol Keys and Spread

The object spread operator does copy symbol keys, unlike for-in and Object.keys, so be aware they carry over when cloning.

const tag = Symbol("tag")
const src = { a: 1, [tag]: 2 }
const copy = { ...src }
console.log(copy[tag])

Storing Private-Like Data

Symbol keys offer a soft form of privacy: the data is reachable only by code holding the symbol, keeping it out of normal property listings.

const balance = Symbol("balance")
function makeAccount() {
  const acct = {}
  acct[balance] = 100
  return { acct: acct, key: balance }
}
const a = makeAccount()
console.log(a.acct[a.key])

Deleting Symbol Properties

You remove a symbol property the same way as any other, using the delete operator with the symbol reference.

const tag = Symbol("tag")
const obj = { [tag]: 5 }
delete obj[tag]
console.log(obj[tag])

Quick Check

Test your symbol key knowledge.

Answer it in the interactive lesson

Recap

Recap: Symbols make safe, hidden object keys.

Use square brackets for computed symbol keys
Skipped by for-in, Object.keys, and JSON
Listed via Object.getOwnPropertySymbols
Great for clash-free metadata

const k = Symbol("k")
const o = { [k]: 7 }
console.log(o[k])

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Frequently Asked Questions

Is the “Symbols as Object Keys” lesson free?

Yes — any 3 lessons of the JavaScript Academy learning path, including “Symbols as Object Keys”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The JavaScript Academy course includes 4 lessons in total.

What will I learn in “Symbols as Object Keys”?

Use symbols for non-colliding properties. You practise JavaScript Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start JavaScript Academy?

No prior experience is required. JavaScript Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 2 of 4.

How long does the “Symbols as Object Keys” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this JavaScript Academy lesson?

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