Create groupAndSum

groupAndSum

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+// Author Tony Germano 
+// Discusion orginated on Mirth Slack from discussion started by nafwa03
+
+function groupAndSum(arr, groupByKeys, sumKeys) {
+  const toGroupKey = o => groupByKeys.map(k => o[k]).join('|')
+  const newResultObject = sourceObj => groupByKeys
+    .map(k => [k, sourceObj[k]])
+    .concat(sumKeys.map(k => [k, 0]))
+    .reduce((targetObj, [k, v]) => (targetObj[k] = v, targetObj), {})
+​
+  const groupMap = Object.create(null)
+  return arr.reduce((result, currentObject) => {
+    const key = toGroupKey(currentObject)
+    const pushSetAndGet = o => (result.push(o), groupMap[key] = o)
+    const resultObject = groupMap[key] || pushSetAndGet(newResultObject(currentObject))
+    sumKeys.forEach(k => { resultObject[k] += currentObject[k] || 0 })
+    return result
+  }, [])
+}