What are Symbols for?

A Symbol is a primitive whose only property is that it is unique. Two symbols are never equal, even with the same description:

Symbol("id") === Symbol("id");   // false

That uniqueness is the whole point: a symbol used as a property key cannot collide with any other key, now or in future.

Avoiding collisions

const cache = Symbol("cache");

function decorate(obj) {
  obj[cache] = expensiveThing();   // cannot clash with anything the object owns
}

This matters when you attach data to objects you do not control — a library adding bookkeeping to user objects, for example. A string key like _cache might overwrite something real.

Hidden from normal iteration

Symbol keys are skipped by for...in, Object.keys, and JSON.stringify:

const o = { visible: 1, [Symbol("hidden")]: 2 };
Object.keys(o);                  // ["visible"]
JSON.stringify(o);               // {"visible":1}
Object.getOwnPropertySymbols(o); // [Symbol(hidden)] — still reachable

This is not privacy — anyone can list them with getOwnPropertySymbols. Use #fields for real privacy. Symbols hide metadata from serialisation, not from determined code.

Well-known symbols hook into the language

The most useful role. These let your object participate in built-in behaviour:

class Range {
  *[Symbol.iterator]() { yield 1; yield 2; }   // now works with for...of
}

class Temp {
  [Symbol.toPrimitive](hint) { return hint === "number" ? 42 : "warm"; }
}

Symbol.iterator, Symbol.asyncIterator, Symbol.toStringTag and Symbol.hasInstance are the ones you will meet.

One more variant

Symbol.for("key") looks up a global registry and returns the same symbol across realms — the opposite of the default behaviour, useful when two independent scripts must agree on a key.