Could you please explain how this piece of code works?

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(' ');
}
titleCase("I'm a little tea pot");

Specifically, what exactly is passed onto to replaceAt (I get that it's passed an index, and a character that's converted to lowercase), but what does replaceAt DO with it?

So, in the first iteration of the loop, it's passed replaceAt(0, i) right? Then what does replaceAt do with this? I just don't get this line:

this.substr(0, index) + character + this.substr(index+character.length)

I've already read this: . I'm here because I don't understand that return statement and what exactly it's doing.

3

4 Answers

Suppose you execute "thisisatest".replaceAt(3, "h").

Then...

1. this.substr(0, index) returns "thi" : ie the first 3 characters of "thisisatest"
2. character returns "h"
3. this.substr(index+character.length) returns "isatest" : ie all characters of "thisisatest", starting at position 4

So, when you combine this, you get "thihisatest"

Lets imagine this easy case:

"0123456789". replaceAt(2/*index*/,"000"/*character*/)

Then this happens:

this.substr(0, index/*2*/)//"01" + character //"000" + this.substr(index/*2*/+)//"56789"

Output:

0100056789

0

The replaceAt function simply takes the index of a character (0 in this case) and replaces it with another character (in this case the uppercase version of the original character. This specific function is just Title Casing a word by replacing the first character with the same character in uppercase.

The line that your questioning, takes a substring of the word before the character at the specificied index this.substr(0,index) since substr is non-inclusive of the last index, appends the specified character + character, and appends a substr of the rest of the word + this.substr(index+character.length)

Example 'testing'.replaceAt(0,testing.charAt(0).toUpperCase()); = '' + 'T' + 'esting' = Testing;

this.substr is a function that operates on a string and returns a 'sub string' of the string. See a tutorial here:

So what replaceAt is doing is operating on a string and replacing the character at the target index, index, with the new substring, character. Indeed the passed character does not have to be only one character but could be multiple, like abcd. It is rather poorly named.

For more detail, using substr(), it is taking the first part of the string from index 0 to index, adding the 'character/string' passed to the function, and then taking the rest of the string from index index+character.length onwards. Note that substr has an optional parameter which is used in the first call (this.substr(0,index)).

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