Grouping objects with lodash or ES5 - arrays

Question

I'm a javascript noobie, I started to use the library lodash and I'm having some problems. Can someone help me? :/
Input:
var a = [{group: 'A', title: 'test1', value: 'test2'},
{group: 'A', title: 'test2', value: 'test3'},
{group: 'B', title: 'test3', value: 'test4'},
{group: 'B', title: 'test1', value: 'test2'},]
Output:
var a = {A: [
{ title: 'test1', value: 'test2'},
{ title: 'test2', value: 'test3'}
],
B: [
{ title: 'test1', value: 'test2'},
{ title: 'test2', value: 'test3'}
],}

Answer 1

You can group with _.groupBy() and then use _.mapValues() on each group to remove the group property from the objects by mapping them to a new object using _.omit().
const a = [{group: 'A', title: 'test1', value: 'test2'},
{group: 'A', title: 'test2', value: 'test3'},
{group: 'B', title: 'test3', value: 'test4'},
{group: 'B', title: 'test1', value: 'test2'}
];
const result = _(a)
.groupBy('group') // group by the group prop
.mapValues((group) => group.map((o) => _.omit(o, 'group'))) // remove the group prop from each item
.value();
console.log(result);
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.4/lodash.min.js"></script>

Answer 2

The lodash documentation for group by is pretty explicit you can find it here.
In your case it would look something like this.
_.groupBy(a, 'group')
You can also do it yourself pretty easily as well using the reduce function. It would look something like this:
let groups = a.reduce((grouping, currentItem) => {
if (!grouping[currentItem.group]) {
grouping[currentItem.group] = []
}
grouping[currentItem.group].push(currentItem);
return grouping;
}, {});
A working example can be found here.
In both cases the result would be an object with the groups as property names.
{
"A": [{
"group": "A",
"title": "test1",
"value": "test2"
}, {
"group": "A",
"title": "test2",
"value": "test3"
}],
"B": [{
"group": "B",
"title": "test3",
"value": "test4"
}, {
"group": "B",
"title": "test1",
"value": "test2"
}]
}