Check If Word Is Valid After Substitutions LeetCode solution in JavaScript

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var isValid = function (s) {
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  const stack = [];
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  for (let c of s) {
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    if (c !== "c") {
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      stack.push(c);
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    } else {
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      if (stack.pop() !== "b") return false;
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      if (stack.pop() !== "a") return false;
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    }
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  }
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  return stack.length === 0;
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};