Closure CavesISSUE #44 of 61

symbols · unique keys

LambdaVSThe Mutator

Lambda saysSymbols are unique, immutable values used as collision-proof object keys.

Symbol("desc") creates a value guaranteed unique — even two symbols with the same description differ. Used as object keys, they never clash with string keys or each other, so libraries use them for "hidden" metadata. Well-known symbols (Symbol.iterator) hook into language behavior.

Power-ups you unlock

Symbol() is always unique
Collision-proof object keys
Symbol-keyed props are hidden from normal enumeration
Well-known symbols hook into language features

The Mutator attacks — common mistakes

Expecting two same-description symbols to be equal
Forgetting symbol keys are skipped by for...in/JSON
Overusing symbols where strings suffice

Boss battleUse a Symbol as an object key and confirm it does not collide with a string key.

Example code

<!doctype html><html><head><meta charset="utf-8"></head>
<body style="background:#06040d;color:#e6e0ff;font-family:monospace;padding:20px"><pre id="o"></pre>
<script>const id=Symbol('id'); const o={[id]:42, id:'string key'};
document.getElementById('o').textContent='symbol='+o[id]+' string='+o.id;</script></body></html>

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