You can easily achieve the result using Set and flat.

const mapedTopicsArray = [
  ["javascript", "reactjs"],
  ["Java", "reactjs"],
];

const topicsArrayMergedWithoutDuplicates  = [...new Set(mapedTopicsArray.flat())];
console.log(topicsArrayMergedWithoutDuplicates );

const mapedTopicsArray = [
                            ['javascript', 'reactjs'],
                            ['Java', 'reactjs'],                   
                         ]


const result = mapedTopicsArray
    .reduce((acc, prev) => acc.concat(prev), [])
    .filter(ifThisItemFirstAppearance)

function ifThisItemFirstAppearance(value, index, self) {
    return self.indexOf(value) === index
}

console.log(result)

First reduce the list of lists into a list, then filter repeated items.

Partial credit to this answer on how to filter duplicates