Using Lodash to sum values by key

Question

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} ] 

Answer 1

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);

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