You could do:

let categorysString = '';
categories.map((item, index) => {
if ( index === 0 ) {
categorysString = categorysString + `${item.categoryName}`;
} else {
categorysString = categorysString + `, ${item.categoryName}`;
}
});

Or you could just do:

return categories.map(category => category.categoryName).join(", ")

🙄

Previous company must have been payed per line...