1//capitalize only the first letter of the string. 
2function capitalizeFirstLetter(string) {
3    return string.charAt(0).toUpperCase() + string.slice(1);
4}
5//capitalize all words of a string. 
6function capitalizeWords(string) {
7    return string.replace(/(?:^|\s)\S/g, function(a) { return a.toUpperCase(); });
8};

1var str = "Hello World!";
2var res = str.toUpperCase();  //HELLO WORLD!

1function capitalizeFirstLetter(string) {
2  return string.charAt(0).toUpperCase() + string.slice(1);
3}
4
5console.log(capitalizeFirstLetter('foo bar bag')); // Foo

1const lower = 'this is an entirely lowercase string';
2const upper = lower.charAt(0).toUpperCase() + lower.substring(1);

1const name = 'flavio'
2const nameCapitalized = name.charAt(0).toUpperCase() + name.slice(1)
3

1With performance metrics...
2
3// 10,889,187 operations/sec
4function capitalizeFirstLetter(string) {
5    return string[0].toUpperCase() + string.slice(1);
6}
7
8// 10,875,535 operations/sec
9function capitalizeFirstLetter(string) {
10    return string.charAt(0).toUpperCase() + string.slice(1);
11}
12
13// 4,632,536 operations/sec
14function capitalizeFirstLetter(string) {
15    return string.replace(/^./, string[0].toUpperCase());
16}
17
18// 1,977,828 operations/sec
19String.prototype.capitalizeFirstLetter = function() {
20    return this.charAt(0).toUpperCase() + this.slice(1);
21}
22