Closure CavesISSUE #48 of 61

typeof vs instanceof

LambdaVSThe Mutator

Lambda saystypeof checks primitive types; instanceof checks an object’s class/prototype chain.

typeof x returns a type string for primitives ("string", "number", "function") — but "object" for arrays, null, and most objects. To tell objects apart, use x instanceof Array (walks the prototype chain) or purpose-built checks like Array.isArray(x). Use the right tool for the right question.

Power-ups you unlock

typeof for primitives and "function"
typeof returns "object" for arrays and null
instanceof checks the prototype chain
Array.isArray for arrays specifically

The Mutator attacks — common mistakes

typeof [] giving "object" and confusing you
instanceof across frames/iframes failing
Using typeof to detect arrays

Boss battleDistinguish an array from a plain object using Array.isArray and typeof.

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], o={};
document.getElementById('o').textContent='isArray(a)='+Array.isArray(a)+' isArray(o)='+Array.isArray(o);</script></body></html>

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