Maximum Number of Removable Characters LeetCode solution in JavaScript

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var maximumRemovals = function (s, p, removable) {
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  let arr = s.split("");
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  const stillFunctions = (k) => {
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    let result = [...arr];
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    for (let i = 0; i < k; i++) {
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      result[removable[i]] = "";
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    }
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    const isSubset = () => {
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      let idx = 0;
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      for (let c = 0; c < p.length; c++) {
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        if (idx > result.length) return false;
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        const next = result.indexOf(p[c], idx);
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        if (next === -1) {
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          return false;
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        }
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        idx = next + 1;
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      }
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      return true;
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    };
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    return isSubset();
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  };
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  let left = 0;
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  let right = removable.length;
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  while (left < right) {
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    // Need to round up the midpoint since we are returning operation LENGTH i.e. ops+1
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    const mid = Math.ceil((left + right) / 2);
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    if (!stillFunctions(mid)) {
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      right = mid - 1;
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    } else {
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      left = mid;
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    }
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  }
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  return left;
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};