Am I using index argument of callback function in .map ( ) wrongly?

Question

I'm trying to capitalise the first letter of each word in a sentence. However, the output I got from my code below is: "I'm a little teapot". I tried finding detailed explanations of how the index argument for the callback function for the map function works but found none. I want to instruct it to capitalise the letter if the element before it is a space. What's wrong with my code?

function titleCase(str) {
str= str.toLowerCase().split("");

var newArr= str.map( function(i,j){

if ( j===0 ){
return i.toUpperCase();}
else if (str[j--]===" "){
return i.toUpperCase();
}
 return i;

});

return newArr.join("");
}


console.log(titleCase("i'm a little tea pot"));

See this for more information regarding [how post increments and decrements work](http://stackoverflow.com/questions/4445706/post-increment-and-pre-increment-concept). — Praveen Kumar Purushothaman, Dec 16 '16 at 11:59
Instead of looping by character, split using spaces, and then loop over the words, capitalizing the first character of each one. — , Dec 16 '16 at 14:18
@torazaburo This looks like a homework gone wrong, and the OP wanted to understand what's wrong and how increments and decrements work. Reopening the question as it is not dupe. — Praveen Kumar Purushothaman, Dec 16 '16 at 14:41

score -1 · Accepted Answer · answered Dec 16 '16 at 11:21

-1

That's crazy here.

else if (str[j--]===" ") {

You are modifying the j value. You shouldn't do that. You should do:

else if (str[j-1]===" ") {

Also, it is a post decrement operator. This is highly dangerous, as it leads to infinite looping. Plus, this always gives the current value, which applies the upper case on the space.

Making that small change fixed it:

function titleCase(str) {
  str = str.toLowerCase().split("");
  var newArr = str.map(function(i, j) {
    if (j === 0) {
      return i.toUpperCase();
    } else if (str[j-1] === " ") {
      return i.toUpperCase();
    }
    return i;
  });
  return newArr.join("");
}

console.log(titleCase("i'm a little tea pot"));

Output:

I'm A Little Tea Pot

answered Dec 16 '16 at 11:21

Praveen Kumar Purushothaman

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Thanks man! I'm completely new to coding so there's a lot I still don't know. One question: how is "j-1" different from "j--"? I thought it's the same. – Emily Dec 16 '16 at 11:28
@Emily Yes, it is confusing. `j--` will loosely equate to `(j = j - 1; return j + 1;)` Now do you understand the problem? – Praveen Kumar Purushothaman Dec 16 '16 at 11:29
Why does it "return j+1"? – Emily Dec 16 '16 at 11:56
@Emily Because it is **post decrement**. It returns the value first, then it decrements. – Praveen Kumar Purushothaman Dec 16 '16 at 11:58
@Emily See this for more information regarding [how post increments and decrements work](http://stackoverflow.com/questions/4445706/post-increment-and-pre-increment-concept). – Praveen Kumar Purushothaman Dec 16 '16 at 11:59
@Emily Any other questions? – Praveen Kumar Purushothaman Dec 16 '16 at 12:09
Just to check, should it be "return j" instead? It returns j first then decrements it by 1. – Emily Dec 16 '16 at 12:13
@Emily I was trying to make you understand. It returns the j value first, and then decrements it. Yes! – Praveen Kumar Purushothaman Dec 16 '16 at 12:14

Am I using index argument of callback function in .map ( ) wrongly?

1 Answers1