function myReplace(str, before, after) {
  // Find index where before is on string
  var index = str.indexOf(before);
  // Check to see if the first letter is uppercase or not
  if (str[index] === str[index].toUpperCase()) {
    // Change the after word to be capitalized before we use it.
    after = after.charAt(0).toUpperCase() + after.slice(1);
  }
  // Now replace the original str with the edited one.
  str = str.replace(before, after);

  return str;
}

// test here
myReplace("A quick brown fox jumped over the lazy dog", "jumped", "leaped");

function myReplace(str, before, after) {
//Create a regular expression object
  var re = new RegExp(before,"gi");
//Check whether the first letter is uppercase or not
  if(/[A-Z]/.test(before[0])){
  //Change the word to be capitalized
    after = after.charAt(0).toUpperCase()+after.slice(1);
     }
     //Replace the original word with new one
  var  newStr =  str.replace(re,after);

 return newStr;
}

// test here
myReplace("A quick brown fox jumped over the lazy dog", "jumped", "leaped");

function myReplace(str, before, after) {

    // create a function that will change the casing of any number of letter in parameter "target"
    // matching parameter "source"
    function applyCasing(source, target) {
        // split the source and target strings to array of letters
        var targetArr = target.split("");
        var sourceArr = source.split("");
        // iterate through all the items of sourceArr and targetArr arrays till loop hits the end of shortest array
        for (var i = 0; i < Math.min(targetArr.length, sourceArr.length); i++){
            // find out the casing of every letter from sourceArr using regular expression
            // if sourceArr[i] is upper case then convert targetArr[i] to upper case
            if (/[A-Z]/.test(sourceArr[i])) {
                targetArr[i] = targetArr[i].toUpperCase();
            }
            // if sourceArr[i] is not upper case then convert targetArr[i] to lower case
            else targetArr[i] = targetArr[i].toLowerCase();
        }
        // join modified targetArr to string and return
        return (targetArr.join(""));
    }


    // replace "before" with "after" with "before"-casing
    return str.replace(before, applyCasing(before, after));
}

// test here
myReplace("A quick brown fox jumped over the lazy dog", "jumped", "leaped");