Group By using Javascript

Question

I have this sample data and manage to use chain of .filter, .map and .reduce to get the total course score for one student. My question is how do I show the total course score for EACH student name? If I remove the filter, it will show the total score for ALL students instead of EACH student.

var data = [
    { name: "John", courseName:"Javascript", courseScore: 85 }, 
    { name: "John", courseName:"C#", courseScore: 72 }, 
    { name: "John", courseName:"SQL", courseScore: 91 },  
    { name: "Mike", courseName:"Javascript", courseScore: 68 }, 
    { name: "Mike", courseName:"C#", courseScore: 80 }, 
    { name: "Mike", courseName:"SQL", courseScore: 79 }  
];


// John's total score for all the courses.
var scoreOfJohn = data
  .filter(x => x.name === 'John')
  .map(x => x.courseScore)
  .reduce((x, y) => x + y);


console.log(scoreOfJohn);

See [JavaScript Group By Array](https://stackoverflow.com/q/12873228/218196) for a start. — Felix Kling, Jan 27 '19 at 09:24
if use object, it can get the Count only, right? what i need is the SUM instead of the Count — Steve, Jan 27 '19 at 09:32

score 1 · Accepted Answer · answered Jan 27 '19 at 09:29

1

As a one liner, grouped to a Map

const grouped = data.reduce((a, b) => a.set(b.name, (a.get(b.name) || 0) + b.courseScore), new Map);
console.log(grouped);

<script>
var data = [
    { name: "John", courseName:"Javascript", courseScore: 85 },
    { name: "John", courseName:"C#", courseScore: 72 },
    { name: "John", courseName:"SQL", courseScore: 91 },
    { name: "Mike", courseName:"Javascript", courseScore: 68 },
    { name: "Mike", courseName:"C#", courseScore: 80 },
    { name: "Mike", courseName:"SQL", courseScore: 79 }
];
</script>

answered Jan 27 '19 at 09:29

baao

71,625
17
143
203

Cool. Thanks a lot. It works. – Steve Jan 27 '19 at 09:37
output is just empty array – brk Jan 27 '19 at 09:43
Open your console and use a not outdated browser @brk – baao Jan 27 '19 at 09:48
it works but can't really understand "(a.get(b.name) || 0) + " – Steve Jan 27 '19 at 09:48

Group By using Javascript

1 Answers1