replaceAtReducer

.js

  function replaceAt(indexArray, string) {
    const func = (string, i) => {
      string[i] = (
        <span className="font-bold text-red" key={i}>
          {string[i]}
        </span>
      );
      return string;
    };

    return indexArray.reduce(func, [...string]);
  }


  const replaceAt = (indexArray, string) =>
    indexArray.reduce(
      (string, i) => {
        string[i] = <b>{string[i]}</b>;
        return string;
      },
      [...string]
    );