Move Pieces to Obtain a String LeetCode solution in JavaScript

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/**
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 * @param {string} start
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 * @param {string} target
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 * @return {boolean}
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 */
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var canChange = function (start, target) {
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  let i = 0;
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  let j = 0;
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  const len = start.length;
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  while (i <= len && j <= len) {
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    while (i < len && start[i] === "_") i++;
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    while (j < len && target[j] === "_") j++;
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    if (i === len && j === len) return true;
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    if (i === len || j === len) return false;
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    if (start[i] !== target[j]) return false;
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    if (start[i] === "L" && j > i) return false;
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    if (start[i] === "R" && j < i) return false;
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    i++;
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    j++;
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  }
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  return false;
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};