JSFiddle - React, Tailwind, and code Playground

HTML

<p>Input:</p>
<pre id=in>[{"a":"1","b":"2","c":"3","d":"4"},null,null,{"a":"9","b":"10","c":"11","d":"12"}]</pre>
<p>Output:</p>
<pre id=out>?</pre>

CSS

pre { font-size: x-small }

JavaScript

var input = $('#in').text();
var inp = JSON.parse(input),
    out = [], i, k, output;
for (i = 0; i < inp.length; i++) {
    out[i] = '';
    if (inp[i]) {
        for (k in inp[i]) {
            if (inp[i].hasOwnProperty(k)) {
                out[i] += k + inp[i][k];
            }
        }
    }
}
output = out.join(',');
$('#out').text(output);