I am doing a simple function. To turn all words first-letter to upper case, but It simply doesn't work, neither display any errors:

function formatTitle(input) {
  var words = input.split(' ');
  for (var i = 0; i < words.length; i++) {
    words[i][0] = words[i][0].toUpperCase();
  };
  return words.join(' ');
};

var newTitle = formatTitle("all words first-letter should be upper case");

document.write(newTitle);

I am doing a simple function. To turn all words first-letter to upper case, but It simply doesn't work, neither display any errors:

function formatTitle(input) {
  var words = input.split(' ');
  for (var i = 0; i < words.length; i++) {
    words[i][0] = words[i][0].toUpperCase();
  };
  return words.join(' ');
};

var newTitle = formatTitle("all words first-letter should be upper case");

document.write(newTitle);

Thanks in advance.

• javascript

• Fancy ES6 approach: [...words[i]].map((a,b)=>!b?a.toUpperCase():a).join('');. – Sebastian Simon Commented Jul 23, 2015 at 13:39
• @Xufox The fancy ES6 approach is much heavier than a fancy regex approach – Denys Séguret Commented Jul 23, 2015 at 15:30

3 Answers 3

The problem is that strings in javascript are immutable. You can't just change a char like this.

A solution would be this:

words[i] = words[i][0].toUpperCase()+words[i].slice(1);

But you could have a simpler and faster code using a regular expression:

return input.replace(/\b\w/g,function(b){ return b.toUpperCase() })

(here with a more plete uppercasing, not just after spaces - if you want to stick to spaces use replace(/(\s+|^)\w/g,function(b){ return b.toUpperCase() }))

Problem

Because

words[i][0] = 'something'

does not update the words[i].

Problem Demo

var myVar = 'abc';

myVar[0] = 'd';

document.write(myVar); // abc

Solution

You can use substr to get the first character and update the value of whole string.

Solution Demo

function formatTitle(input) {
  var words = input.split(' ');
  for (var i = 0; i < words.length; i++) {
    words[i] = words[i].substr(0, 1).toUpperCase() + words[i].substr(1);
  }
  return words.join(' ');
}

var newTitle = formatTitle("all words first-letter should be upper case");

document.write(newTitle);

How wrote Denis the reason is that strings in javascript are immutable (numbers and booleans are also immutable).

Another very simple solution for Upperize the first char of a string is:

function firstUpper(word) {
     return word.charAt(0).toUpperCase() + word.substring(1);
};

I suggest also to read this post: Understanding Javascript immutable variable

Hope this help