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var sortByBits = function (arr) {
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  const map = {};
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  for (let n of arr) {
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    let counter = 0,
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      item = n;
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    while (item > 0) {
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      counter += item & 1; //increment counter if the lowest (i.e. the rightest) bit is 1
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      item = item >> 1; //bitwise right shift (here is equivalent to division by 2)
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    }
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    map[n] = counter;
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  }
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  return arr.sort((a, b) => map[a] - map[b] || a - b); //sort by number of 1 bits; if equal, sort by value
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};