Why won't array[i].charAt(0).toUpperCase() pass on to array[i][0]

Question

I am trying to make a javascript program that takes a string and capitalizes the first letter of every word and makes every other character lowercase.

function titleCase(str) {
  str = str.toLowerCase();
  var array = str.split(" ");
  for(var i =0; i< array.length ; i++){
    array[i][0] = array[i].charAt(0).toUpperCase();

  } 
  var finalString = array.join(" ")
  return finalString ; 
}

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

For some reason array[i].charAt(0).toUpperCase(); won't pass it's value to array[i][0]. This ends up making it return the string with just all lowercase letters instead of having the first letter of each word being capitalized.

You can't index into the string like that with `array[i][0]` — Sean, Sep 23 '15 at 21:00

CollinD · Answer 1 · 2019-09-27T15:54:13.433

Sounds like you want to do something more like this:

function titleCase(str) {
  str = str.toLowerCase();
  var array = str.split(" ");
  for(var i =0; i< array.length ; i++){
    //array[i] is an immutable string. So we need to rebuild it instead.
    array[i] = array[i].charAt(0).toUpperCase() + array[i].substring(1);
  } 
  var finalString = array.join(" ")
  return finalString; 
}

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

Strings can be read as char arrays using bracket notation. However, you cannot change a particular character that way since strings are not mutable objects.

score 3 · Answer 2 · answered Sep 23 '15 at 21:01

JavaScript strings are immutable, so you cannot actually change individual characters by indexing them using the [] operator. Here is a fixed version that uses substring to build the final string instead:

Live Demo:

function titleCase(str) {
  str = str.toLowerCase();
  var array = str.split(" ");
  for(var i =0; i< array.length ; i++){
    array[i] = array[i].charAt(0).toUpperCase() + array[i].substring(1);

  } 
   var finalString = array.join(" ")
  return finalString ; 
}

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

JSFiddle Version: https://jsfiddle.net/rakdtpbb/

score 0 · Answer 3 · edited Sep 24 '15 at 07:42

0

The toUpperCase() method returns the value of the string converted to uppercase, but does not affect the value of the string itself.

edited Sep 24 '15 at 07:42

Anders

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answered Sep 23 '15 at 21:04

Stella

139
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Thanks that solved my problem. I did not realize you cannot index individual characters. – vin42tau Sep 25 '15 at 01:28

Why won't array[i].charAt(0).toUpperCase() pass on to array[i][0]

3 Answers3