Title Case a Sentence Solution Questions

JavaScript
1

Isn’t this.substr(0, index) always this.substr(0, 0)? Why is it necessary? I attempted to use Javascript Tutor to see what’s happening in the user-defined replaceAt function, but I can’t see why this.substr(0,0) is needed because JT doesn’t show exactly what’s happening. The code is quickly run and spits out the result.

String.prototype.replaceAt = function(index, character) {
  return (
    this.substr(0, index) + character + this.substr(index + character.length)
  );
};

function titleCase(str) {
  var newTitle = str.split(" ");
  var updatedTitle = [];
  for (var st in newTitle) {
    updatedTitle[st] = newTitle[st]
      .toLowerCase()
      .replaceAt(0, newTitle[st].charAt(0).toUpperCase());
  }
  return updatedTitle.join(" ");
}
2

Hello there,

Why do you assume index is always equal to 0?

3

because of the arguments in replaceAt.

string.replaceAt(0, newTitle[st].charAt(0).toUpperCase());

The first argument index is 0.

4

In this case, yes. But, the function is made in such a way that you could choose to replace at a different index, and are not limited to always replacing the first.

5

If you don’t mind, would you be able to walk me through what happens to the first word in the sentence, “I’m” ?

6

I could, but here is an excellent resource for this:
http://pythontutor.com/javascript.html#mode=edit

8

Ah. Well, first thing to recognise is what is being passed to the replaceAt function:
In this case:

.replaceAt(0, newTitle[st].charAt(0).toUpperCase())
//Same as
.replaceAt(0, 'I')

Now, what does the substr method do: https://www.w3schools.com/jsref/jsref_substr.asp
In this case, the this keyword is referencing the object the replaceAt function was called on:
the lowerCase value of newTitle[st] == "I"

Now, substr(0,0) returns nothing. Same with substr(1) on an object of length 1.

So, replaceAt returns "I".

Hope that helps