TypeError problem

JavaScript
1 
function myReplace(str, before, after) {
  let index=str.indexOf(before)
  if(str[index]===str[index].toUpperCase()){
    after=after.charAt(0).toUpperCase()+after.slice(1)
  }
  console.log(after)
}

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

The output shows typeError
TypeError: Cannot read properties of undefined (reading ‘toUpperCase’)

Can someone explain why this is happening?

2

The string “jumped” is not found in “A quick brown fox umped over the lazy dog” so index is -1, so str[index] is undefined.

1 Like
3

even if its ‘jumped’, still same error shows up if i console.log it

4

What this error is saying is, “We are trying read a property/method called ‘toUpperCase’ but the place we are looking is a variable that evaluates to ‘undefined’. That is illegal.”

That means that either str[index] is undefined or after.charAt(0) is.

I would put in some log statements like this:

function myReplace(str, before, after) {
  let index=str.indexOf(before)
  console.log('index', index)
  console.log('str[index]', str[index]) 
  console.log('after.charAt(0)', after.charAt(0))
  if(str[index]===str[index].toUpperCase()){
    after=after.charAt(0).toUpperCase()+after.slice(1)
  }
  console.log(after)
}

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

This is what @colinthornton was talking about. If I correct the text, I don’t get the error.

And please don’t make changes to code after they’ve been discussed - it makes it confusing to follow the thread.

1 Like
5

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