JSFiddle - React, Tailwind, and code Playground

HTML

<div id=answer></div>

CSS

body {
   font-family: monospace;
   padding: 20px;
   }

JavaScript

var json = {"result":[["abc","de"],["fgh"],["ij","kl"]]};

function concatArrays(a, b) { return a.concat(b); }
json.result = json.result.reduce(concatArrays);

$('#answer').text(JSON.stringify(json));