Most efficient method to groupby on an array of objects

Question

What is the most efficient way to groupby objects in an array?

For example, given this array of objects:

[      { Phase: "Phase 1", Step: "Step 1", Task: "Task 1", Value: "5" },     { Phase: "Phase 1", Step: "Step 1", Task: "Task 2", Value: "10" },     { Phase: "Phase 1", Step: "Step 2", Task: "Task 1", Value: "15" },     { Phase: "Phase 1", Step: "Step 2", Task: "Task 2", Value: "20" },     { Phase: "Phase 2", Step: "Step 1", Task: "Task 1", Value: "25" },     { Phase: "Phase 2", Step: "Step 1", Task: "Task 2", Value: "30" },     { Phase: "Phase 2", Step: "Step 2", Task: "Task 1", Value: "35" },     { Phase: "Phase 2", Step: "Step 2", Task: "Task 2", Value: "40" } ] 

I’m displaying this information in a table. I’d like to groupby different methods, but I want to sum the values.

I’m using Underscore.js for its groupby function, which is helpful, but doesn’t do the whole trick, because I don’t want them “split up” but “merged”, more like the SQL group by method.

What I’m looking for would be able to total specific values (if requested).

So if I did groupby Phase, I’d want to receive:

[     { Phase: "Phase 1", Value: 50 },     { Phase: "Phase 2", Value: 130 } ] 

And if I did groupy Phase / Step, I’d receive:

[     { Phase: "Phase 1", Step: "Step 1", Value: 15 },     { Phase: "Phase 1", Step: "Step 2", Value: 35 },     { Phase: "Phase 2", Step: "Step 1", Value: 55 },     { Phase: "Phase 2", Step: "Step 2", Value: 75 } ] 

Is there a helpful script for this, or should I stick to using Underscore.js, and then looping through the resulting object to do the totals myself?

Answer 1

If you want to avoid external libraries, you can concisely implement a vanilla version of groupBy() like so:

var groupBy = function(xs, key) {   return xs.reduce(function(rv, x) {     (rv[x[key]] = rv[x[key]] || []).push(x);     return rv;   }, {}); };  console.log(groupBy(['one', 'two', 'three'], 'length'));  // => {3: ["one", "two"], 5: ["three"]}