Javascript bold or replace character in string at index position

.js

// Working example
// https://codesandbox.io/s/lr5zjxrn67

// Method #1
function replaceAt(string, indexArray) {
  let newString = [...string];

  for (let i = 0; i < indexArray.length; i++) {
    newString = Object.assign(newString, {
      [indexArray[i]]: <b>{newString[indexArray[i]]}</b>
    });
  }

  return newString;
}


//Method #2
function replaceAt(string, indexArray) {
  const newString = [...string];

  return newString.map(i => {
    indexArray.map(i => {
      return (newString[i] = <b>{newString[i]}</b>);
    });
    return i;
  });
}