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Well-known Symbols

Well-known symbols are static properties of Symbol (written @@iterator in older spec text) that the engine consults during an operation. A method stored under one of these keys customizes that operation for your objects.

The well-known symbols
Symbol Consulted by
iterator, asyncIterator for...of, for await...of (Iterables and Iterators)
match, matchAll, replace, search, split Same-named string methods (Regular Expressions)
hasInstance instanceof
toPrimitive, toStringTag Type conversion, Object.prototype.toString()
species map(), then()... (avoid: may be removed)
isConcatSpreadable, unscopables concat(), with statements
dispose, asyncDispose using, await using (ES2027)
Customizing type conversion and the toString() tagJavaScriptLive
class Celsius {
  constructor(deg) { this.deg = deg; }
  [Symbol.toPrimitive](hint) { return hint === 'string' ? `${this.deg}°C` : this.deg; }
  get [Symbol.toStringTag]() { return 'Celsius'; }
}
const t = new Celsius(21);
console.log(`${t}`, t + 1, Object.prototype.toString.call(t));