Maximum Width Ramp LeetCode solution in JavaScript

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var maxWidthRamp = function (nums) {
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  let stack = [],
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    ans = 0;
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  for (let i = 0; i < nums.length; i++) {
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    let index = lower_bound(stack, i);
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    ans = Math.max(ans, i - index);
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    if (!stack.length || nums[i] < nums[stack[stack.length - 1]]) stack.push(i);
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  }
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  return ans;
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  function lower_bound(arr, index) {
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    if (!arr.length) return index;
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    let low = 0,
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      high = arr.length - 1;
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    while (low < high) {
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      let mid = Math.floor((low + high) / 2);
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      if (nums[arr[mid]] <= nums[index]) high = mid;
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      else low = mid + 1;
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    }
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    return nums[arr[low]] <= nums[index] ? arr[low] : index;
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  }
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};