4Sum II LeetCode solution in JavaScript

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var fourSumCount = function (nums1, nums2, nums3, nums4) {
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  let result = 0;
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  let sumMap = {};
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  for (let i = 0; i < nums1.length; i++) {
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    for (let j = 0; j < nums2.length; j++) {
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      let sum = nums1[i] + nums2[j];
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      if (!sumMap[sum]) {
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        sumMap[sum] = 0;
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      }
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      sumMap[sum] += 1;
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    }
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  }
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  for (let k = 0; k < nums3.length; k++) {
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    for (let l = 0; l < nums4.length; l++) {
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      let sum = nums3[k] + nums4[l];
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      if (sumMap[-sum]) {
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        result += sumMap[-sum];
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      }
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    }
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  }
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  return result;
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};