JSFiddle - React, Tailwind, and code Playground

JavaScript

var json = '{"rows":[{"key":["zeit.de"],"value":98},{"key":["google.com"],"value":49},{"key":["spiegel.de"],"value":20},{"key":["spiegel.de"],"value":12},{"key":["spiegel.de"],"value":20},{"key":["spiegel.de"],"value":12},{"key":["netmng.com"],"value":49},{"key":["zeit.de"],"value":300}]}';
var obj = JSON.parse(json);

var newObj = {};
for(i in obj['rows']){
 var item = obj['rows'][i];
    if(newObj[item.key[0]] === undefined){
        newObj[item.key[0]] = 0;
    }
    newObj[item.key[0]] += item.value;
}

var result = {};
result.rows = [];
for(i in newObj){
    result.rows.push({'key':i,'value':newObj[i]});
}
console.log(result);