Using Lodash to sum values by key

How can I sum the values in objects which share a common key? I need to use Lodash for this because I need good performance if these arrays get huge.

var prjMgrValues = [
   {"proj_mgr":"Jack ProjManager","submitted_dollars":12000},
   {"proj_mgr":"Jack ProjManager","submitted_dollars":750000},
   {"proj_mgr":"Joe ProjManager","submitted_dollars":45000}
]

I'm looking for an output of

[
  {"proj_mgr":"Jack ProjManager","submitted_dollars":762000},
  {"proj_mgr":"Joe ProjManager","submitted_dollars":45000}
]

Answers:

Answer

This is a case of reduction for each unique element.

I always use _.groupBy and then _.map the result to an array after applying the reduction. In this case the reduction operation is _.sumBy.

var prjMgrValues = [
   {"proj_mgr":"Jack ProjManager","submitted_dollars":12000},
   {"proj_mgr":"Jack ProjManager","submitted_dollars":750000},
   {"proj_mgr":"Joe ProjManager","submitted_dollars":45000}
];

var output =
  _(prjMgrValues)
    .groupBy('proj_mgr')
    .map((objs, key) => ({
        'proj_mgr': key,
        'submitted_dollars': _.sumBy(objs, 'submitted_dollars') }))
    .value();

console.log(output);
<script src="https://cdn.jsdelivr.net/lodash/4.14.1/lodash.min.js"></script>

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