*1. Code:*

replaceString(txt, keyword){
   let returnTxt = txt;

   if (keyword) {
      const parts = txt.split(keyword);
      const partLen = parts.length;
      const result = [];

      for (let i = 0; i < partLen; i += 1) {
        result.push(parts[i]);

        if (partLen - i > 1)
          result.push(<span className="highlight-txt">{keyword}</span>);
      }
      returnTxt = result;
    }

    return returnTxt;
  }

*2. Example:*
const text = "the lazy fox jumps over the quick dog";
var stringJXS = replaceString(text, 'over');

// result equipvalent to:   "the lazy fox jumps <span className="highlight-txt">over</span> the quick dog"