 <p>If you want to modify the initial object - it would be enough to use <code>Object.keys</code> and <code>Array.forEach</code> functions:</p> <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false"> <div class="snippet-code"> <pre class="snippet-code-js lang-js prettyprint-override"><code>var obj = { "page": [ "POST", "DELETE" ], "news": [ "PUT" ] }; Object.keys(obj).forEach(function(k) { obj[k] = obj[k].join(","); }); console.log(JSON.stringify(obj, 0, 4));</code></pre> </div> </div> </p> <p><strong>Additional solution</strong> for another complex case:</p> <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false"> <div class="snippet-code"> <pre class="snippet-code-js lang-js prettyprint-override"><code>var obj = { "page": { "POST": [ "POST" ], "PUT": 122 }, "news": { "PUT": [ "PUT" ] } }; Object.keys(obj).forEach(function (k) { var innerKeys = Object.keys(obj[k]), items = []; innerKeys.forEach(function(key) { items.push((Array.isArray(obj[k][key]))? key : key + ":" + obj[k][key]); }); obj[k] = items.join(","); }); console.log(JSON.stringify(obj, 0, 4));</code></pre> </div> </div> </p>