Symbols in JavaScript

Unique keys that never collide, and the well-known ones with meaning.

const INTERNAL = Symbol("internal");

const tour = { slug: "patterns", [INTERNAL]: { cached: true } };
console.log(Object.keys(tour), JSON.stringify(tour));
console.log(tour[INTERNAL].cached, Object.getOwnPropertySymbols(tour).length);

console.log(Symbol("a") === Symbol("a"), Symbol.for("a") === Symbol.for("a"));

class Range {
  constructor(to) { this.to = to; }
  *[Symbol.iterator]() { for (let i = 1; i <= this.to; i += 1) yield i; }
  get [Symbol.toStringTag]() { return "Range"; }
  [Symbol.toPrimitive](hint) { return hint === "number" ? this.to : "range"; }
}

const range = new Range(3);
console.log([...range], `${range}`, +range, Object.prototype.toString
  .call(range));

How it works

1. Symbol(\u0022x\u0022) is unique; the description is only a label.
2. A symbol key is skipped by JSON and by Object.keys.
3. The well-known symbols hook into language behavior.

Keywords and builtins used here

• JSON
• Object
• Symbol
• class
• const
• constructor
• for
• let
• return
• this
• yield

The run, in numbers

Lines

18

Characters to type

676

Tokens

215

Three-star pace

110 tpm

At the three-star pace of 110 tokens a minute, this run takes about 117 seconds.

Type this snippet

Step 2 of 4 in Objects under the hood, step 13 of 16 in Functions & patterns.

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