Closure CavesISSUE #43 of 61

prototypes · the old inheritance

LambdaVSThe Mutator

Lambda saysPrototypes are the real inheritance mechanism classes are built on.

Every object has a hidden link (__proto__) to a prototype object; property lookups walk this prototype chain until found or null. Classes just configure prototypes for you. Understanding this explains why all array instances share map (it lives on Array.prototype) and why you can extend built-ins (carefully).

Power-ups you unlock

Objects link to a prototype (the chain)
Lookups walk the chain until found
Classes configure prototypes under the hood
Shared methods live on the prototype

The Mutator attacks — common mistakes

Modifying built-in prototypes (Array.prototype) in libraries
Confusing __proto__ with prototype
Deep prototype chains slowing lookups

Boss battleShow that an array instance finds map() on Array.prototype.

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 a=[1,2];
document.getElementById('o').textContent='map on prototype: '+(Object.getPrototypeOf(a)===Array.prototype);</script></body></html>

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