JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://underscorejs.org/underscore-min.js"></script>

CSS

p{
    margin:10px;
    }

JavaScript

abcArr = [["A", 10], ["B", 20], ["A",30],["C",40]]

var reduced = _.map(
    _.groupBy(abcArr, function(x){return x[0]}),
    function(x){return _.reduce(x,
        function(x,y){return [y[0], x[1]+y[1]];},[,0])});

$(document.body).append("<p>"+JSON.stringify(reduced))