tech(array): array reduce by example

Author: shangan23

array/reduce.js

@@ -0,0 +1,62 @@
+const array1 = [1, 2, 3, 4]
+const log = console.log
+
+log()
+
+const reducer = (accumulator, currentValue) => accumulator + currentValue
+
+// 1 + 2 + 3 + 4
+log('1 + 2 + 3 + 4 = ', array1.reduce(reducer),0)
+// expected output: 10
+
+// 5 + 1 + 2 + 3 + 4
+log('5 + 1 + 2 + 3 + 4 = ', array1.reduce(reducer, 5))
+// expected output: 15
+
+log()
+// Counting instances of values in an object
+let names = ['Alice', 'Bob', 'Tiff', 'Bruce', 'Alice']
+
+let countedNames = names.reduce(function (allNames, name) {
+  if (name in allNames) {
+    allNames[name]++
+  } else {
+    allNames[name] = 1
+  }
+  return allNames
+}, {})
+
+log('countedNames', countedNames)
+log()
+// countedNames is:
+// { 'Alice': 2, 'Bob': 1, 'Tiff': 1, 'Bruce': 1 }
+
+// Grouping objects by a property
+let people = [
+  { name: 'Alice', age: 21 },
+  { name: 'Max', age: 20 },
+  { name: 'Jane', age: 20 },
+]
+
+function groupBy(objectArray, property) {
+  return objectArray.reduce(function (acc, obj) {
+    let key = obj[property]
+    if (!acc[key]) {
+      acc[key] = []
+    }
+    acc[key].push(obj)
+    return acc
+  }, {})
+}
+
+let groupedPeople = groupBy(people, 'age')
+log('groupedPeople', groupedPeople)
+log()
+// groupedPeople is:
+// {
+//   20: [
+//     { name: 'Max', age: 20 },
+//     { name: 'Jane', age: 20 }
+//   ],
+//   21: [{ name: 'Alice', age: 21 }]
+// }