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This is a test app which may provide someone to accomplish payment operation.
Powered by React, bootstrap.
Click the follow link to see that.

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Refactoring task.

1. Previous function:

function  func(s, a, b) {
  if (s.match(/^$/)) {
    return -1
  }
  var  i = s.length - 1
  var  aIndex = -1
  var  bIndex = -1
   
  while (aIndex == -1 && bIndex == -1 && i > 0) {
    if (s.substring(i, i + 1) == a) {
      aIndex = i     
    }
    if (s.substring(i, i + 1) == b) {
      bIndex = i
    }
    i = i - 1
  }

  if (aIndex != -1) {
    if (bIndex == -1) {
      return  aIndex
    } else {
      return  Math.max(aIndex, bIndex)
      }
  }
  if (bIndex != -1) {
    return  bIndex
  } else {
    return -1
    }
  }

2. New function:

function  myFunc(s, a, b) {
  if (typeof  s !== 'string' || s.length == 0){
   return -1
  }
  let  index = -1

  for (let  i = 0; i < s.length; i++) {
    if (s[i] == a || s[i] == b) {
      index = i
    }
  }
    return  index
}