ES6 Javascript (React) code that maps and filters JSON data
You can use a for..of loop to avoid using .map() twice and Set

const files = Object.values(gist.files);
const unique_tags = []; 

for (let file of files) {
  let tag = file.language ? file.language : 'Plain Text';
  if (unique_tags.indexOf(&lt;Tag value={tag}/&gt;) === -1) {
    unique_tags = [...unique_tags, &lt;Tag value={tag}/&gt;]
  }
}