Lady, EVOLVED 
javascript
very close to getting `dense()´ to work (as a proxy not a copying function) but alas it is time for bed
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javascript
const s = [,,,];
2 in s; // false
const d = dense(s);
2 in d; // true
d[2]; // undefined
s[2] = "value";
s[2]; // "value"
d[2]; // "value"
javascript
this is almost certainly a profoundly difficult-to-optimize way of doing this but i love giving browser manufacturers impossible problems to solve