JSFiddle - React, Tailwind, and code Playground

JavaScript

var array1 = [[5,10],[6,10],[7,10],[8,10],[9,10]];
var array2 = [[1,10],[2,10],[3,10],[4,10],[5,40],[6,40]];


var sums = {};

[array1, array2].forEach(function(array) {
    array.forEach(function(pair) {
        sums[pair[0]] = pair[1] + (sums[pair[0]] || 0);
    });
});

var results = [];
for(var key in sums) {
    results.push([key, sums[key]]);
}
                  
alert(results);