Conditions coerce, and the surprises are real: [] and {} are truthy, so if (arr) is true even for an empty array (check arr.length). "0" is truthy but 0 is falsy. Memorize the six falsy values and reach for explicit checks when intent matters.
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<script>const arr=[]; const truthy=!!arr; const empty=arr.length===0;
document.getElementById('o').textContent='[] truthy='+truthy+' empty='+empty;</script></body></html>