function toTitleCase(str) {

  return str.toLowerCase().split(' ').map(function(word) {

    return word.charAt(0).toUpperCase() + word.slice(1);

  }).join(' ');

}

​

// Example usage

var sentence = "this is an example sentence";

var converted = toTitleCase(sentence);

console.log(converted);  // Output: This Is An Example Sentence