Script SpringsISSUE #14 of 29

booleans · truthy · falsy

SparkyVSNaN the Trickster

Sparky saysBooleans are true/false; many values coerce to them as "truthy" or "falsy".

Conditions need a boolean, so JS coerces. The falsy values are exactly: false, 0, "", null, undefined, NaN (and 0n). Everything else is truthy — including "0", [], and {}. Boolean(x) or !!x converts explicitly.

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Falsy: false, 0, "", null, undefined, NaN
Everything else is truthy (incl. [] and {})
Boolean(x) or !!x converts explicitly
Conditions coerce values to boolean

NaN the Trickster attacks — common mistakes

Thinking "0" or [] is falsy (they’re truthy)
if (x = 5) assignment instead of comparison
Relying on loose truthiness where a strict check is clearer

Boss battleLog the truthiness of 0, "", "0", [], and {} with !! — note which surprise you.

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 v=[0,'','0',[],{}].map(x=>JSON.stringify(x)+' → '+!!x);
document.getElementById('o').textContent=v.join('\n');</script></body></html>

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