Try this:

function toTitleCase(str) {
  return str.replace(
    /\w\S*/g,
    function(txt) {
      return txt.charAt(0).toUpperCase() + txt.substr(1).toLowerCase();
    }
  );
}

<form>
  Input:
  <br /><textarea name="input" onchange="form.output.value=toTitleCase(this.value)" onkeyup="form.output.value=toTitleCase(this.value)"></textarea>
  <br />Output:
  <br /><textarea name="output" readonly onclick="select(this)"></textarea>
</form>

If a CSS solution meets your needs, you can apply the text-transform CSS style to your controls:

text-transform: capitalize;

Just be aware that this will transform:
hello world to Hello World
HELLO WORLD to HELLO WORLD (no change)
emily-jane o'brien to Emily-jane O'brien (incorrect)
Maria von Trapp to Maria Von Trapp (incorrect)

A slightly more elegant way, adapting Greg Dean's function:

String.prototype.toProperCase = function () {
    return this.replace(/\w\S*/g, function(txt){return txt.charAt(0).toUpperCase() + txt.substr(1).toLowerCase();});
};

Call it like:

"pascal".toProperCase();

Here's my version, IMO it's easy to understand and elegant too.

const str = "foo bar baz";
const newStr = str.split(' ')
   .map(w => w[0].toUpperCase() + w.substring(1).toLowerCase())
   .join(' ');
console.log(newStr);

Here’s my function that converts to title case but also preserves defined acronyms as uppercase and minor words as lowercase:

String.prototype.toTitleCase = function() {
  var i, j, str, lowers, uppers;
  str = this.replace(/([^\W_]+[^\s-]*) */g, function(txt) {
    return txt.charAt(0).toUpperCase() + txt.substr(1).toLowerCase();
  });

  // Certain minor words should be left lowercase unless 
  // they are the first or last words in the string
  lowers = ['A', 'An', 'The', 'And', 'But', 'Or', 'For', 'Nor', 'As', 'At', 
  'By', 'For', 'From', 'In', 'Into', 'Near', 'Of', 'On', 'Onto', 'To', 'With'];
  for (i = 0, j = lowers.length; i < j; i++)
    str = str.replace(new RegExp('\\s' + lowers[i] + '\\s', 'g'), 
      function(txt) {
        return txt.toLowerCase();
      });

  // Certain words such as initialisms or acronyms should be left uppercase
  uppers = ['Id', 'Tv'];
  for (i = 0, j = uppers.length; i < j; i++)
    str = str.replace(new RegExp('\\b' + uppers[i] + '\\b', 'g'), 
      uppers[i].toUpperCase());

  return str;
}

For example:

"TO LOGIN TO THIS SITE and watch tv, please enter a valid id:".toTitleCase();
// Returns: "To Login to This Site and Watch TV, Please Enter a Valid ID:"

I prefer the following over the other answers. It matches only the first letter of each word and capitalises it. Simpler code, easier to read and less bytes. It preserves existing capital letters to prevent distorting acronyms. However you can always call toLowerCase() on your string first.

function title(str) {
  return str.replace(/(^|\s)\S/g, function(t) { return t.toUpperCase() });
}

You can add this to your string prototype which will allow you to 'my string'.toTitle() as follows:

String.prototype.toTitle = function() {
  return this.replace(/(^|\s)\S/g, function(t) { return t.toUpperCase() });
}

Example:

String.prototype.toTitle = function() {
  return this.replace(/(^|\s)\S/g, function(t) { return t.toUpperCase() });
}

console.log('all lower case ->','all lower case'.toTitle());
console.log('ALL UPPER CASE ->','ALL UPPER CASE'.toTitle());
console.log("I'm a little teapot ->","I'm a little teapot".toTitle());

You could immediately toLowerCase the string, and then just toUpperCase the first letter of each word. Becomes a very simple 1 liner:

function titleCase(str) {
  return str.toLowerCase().replace(/\b(\w)/g, s => s.toUpperCase());
}

console.log(titleCase('iron man'));
console.log(titleCase('iNcrEdible hulK'));

Benchmark

TL;DR

The winner of this benchmark is the plain old for loop:

function titleize(str) {
    let upper = true
    let newStr = ""
    for (let i = 0, l = str.length; i < l; i++) {
        // Note that you can also check for all kinds of spaces  with
        // str[i].match(/\s/)
        if (str[i] == " ") {
            upper = true
            newStr += str[i]
            continue
        }
        newStr += upper ? str[i].toUpperCase() : str[i].toLowerCase()
        upper = false
    }
    return newStr
}
// NOTE: you could beat that using charcode and string builder I guess.