Group the People Given the Group Size They Belong To LeetCode solution in JavaScript

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/**
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 * @param {number[]} groupSizes
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 * @return {number[][]}
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 */
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var groupThePeople = function (groupSizes) {
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  let indices = [],
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    result = [];
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  groupSizes.forEach((x, idx) => {
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    if (indices[x]) indices[x].push(idx);
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    else indices[x] = [idx];
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    if (indices[x].length === x) {
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      result.push(indices[x]);
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      indices[x] = undefined;
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    }
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  });
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  return result;
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};