gistfile1.txt

function replace(str, before, after) {
 var arr = str.split("")
 var newArr = []
 
 for(i=0;i<arr.length;i++){
   if (arr[i]!=before){
     newArr.push(arr[i])
   }else{  
     newArr.push(after)
   } 
   var newStr = newArr.join("") 
 }
  return newStr
}
replace("A quick brown fox jumped over the lazy dog", "jumped", "leaped");