GroupByTwoKeysAndPivotValuesIntoArray.js

// Produces one row for each unique combination of KeyA and KeyB with
// a Values property that is an array of values that match the
// given key combination.

// Requires: underscore/lodash

var raw = [
 { KeyA: "A", KeyB: 11, Value: 100 },
 { KeyA: "A", KeyB: 11, Value: 200 },
 { KeyA: "A", KeyB: 22, Value: 300 },
 { KeyA: "A", KeyB: 33, Value: 350 },
 { KeyA: "B", KeyB: 11, Value: 400 },
 { KeyA: "B", KeyB: 11, Value: 500 },
 { KeyA: "B", KeyB: 22, Value: 600 },
 { KeyA: "B", KeyB: 22, Value: 700 }
];

var grouped = _.chain(raw)
 .groupBy(function(x) { return x.KeyA + '_' + x.KeyB; })
 .map(function(x) {
 var first = x[0]; // At least one element always exists
 var values = x.map(function(v) { return v.Value; });
 return { KeyA: first.KeyA, KeyB: first.KeyB, Values: values };
 })
 .value()

console.log(grouped);

// Produces:
// [
// { KeyA: "A", KeyB: 11, Values: [ 100, 200 ] },
// { KeyA: "A", KeyB: 22, Values: [ 300 ] },
// ...
// ]