How can I make the first letters of the capitalized words appear?

The shortest implementation to capitalize words within a string is the following using the Arrow functions of ES6:

'your string'.replace(/\b\w/g, l => l.toUpperCase())
// => 'Your String'

Implementation compatible with ES5:

'your string'.replace(/\b\w/g, function(l){ return l.toUpperCase() })
// => 'Your String'

The regular expression matches the first letter of each word within the given string and transforms only that uppercase letter:

\ b matches a word limit (the beginning or end of the word);
\ w matches the following metacharacters [a-zA-Z0-9].

Using, replace (), regular expressions and ES6 arrow functions

console.log('kdz stack overflow'.replace(/\b[a-z]/g,c=>c.toUpperCase()));

Let's make a solution in Spanish with regular expressions.

  

About the letters of Spanish:
  For regular expressions in JavaScript, the ñ and the accents are not word characters 1 .
1 : Not included in \w , and therefore \b misses them

In JavaScript 2 , we have to match the previous character (or the beginning of the string) and include it in the replacement, followed by a lowercase letter.
2 : Because in JavaScript there are no backward inspections ( lookbehinds ), as there are in other dialects of regex.

To capitalize the lowercase at the beginning of each word:

const re = /(^|[^A-Za-zÁÉÍÓÚÜÑáéíóúüñ])([a-záéíóúüñ])/g;
texto.replace(re, function(match, caracterPrevio, minuscula) {
    return caracterPrevio + minuscula.toLocaleUpperCase(['es', 'gl', 'ca', 'pt', 'en']);
});

And, in addition, we can take to lowercase all the rest, doing:

• that also matches initial capitals (not to replace),
• and with intermediate capital letters (to lower case).

Solution:

function titleCase(texto) {
    const re = /(^|[^A-Za-zÁÉÍÓÚÜÑáéíóúüñ])(?:([a-záéíóúüñ])|([A-ZÁÉÍÓÚÜÑ]))|([A-ZÁÉÍÓÚÜÑ]+)/gu;
    return texto.replace(re,
        (m, caracterPrevio, minuscInicial, mayuscInicial, mayuscIntermedias) => {
            const locale = ['es', 'gl', 'ca', 'pt', 'en'];
            if (mayuscIntermedias)
                return mayuscIntermedias.toLocaleLowerCase(locale);
            return caracterPrevio
                 + (minuscInicial ? minuscInicial.toLocaleUpperCase(locale) : mayuscInicial);
        }
    );
}

Description:

/(^|[^A-Za-zÁÉÍÓÚÜÑáéíóúüñ])(?:([a-záéíóúüñ])|([A-ZÁÉÍÓÚÜÑ]))|([A-ZÁÉÍÓÚÜÑ]+)/gu

They are 2 alternatives in a OR : /1|2/gu

Demo:

//Leva a mayúsculas la primera letra de cada palabra
function titleCase(texto) {
    const re = /(^|[^A-Za-zÁÉÍÓÚÜÑáéíóúüñ])(?:([a-záéíóúüñ])|([A-ZÁÉÍÓÚÜÑ]))|([A-ZÁÉÍÓÚÜÑ]+)/gu;
    return texto.replace(re,
        (m, caracterPrevio, minuscInicial, mayuscInicial, mayuscIntermedias) => {
            const locale = ['es', 'gl', 'ca', 'pt', 'en'];
            //Son letras mayúsculas en el medio de la palabra
            // => llevar a minúsculas.
            if (mayuscIntermedias)
                return mayuscIntermedias.toLocaleLowerCase(locale);
            //Es la letra inicial de la palabra
            // => dejar el caracter previo como está.
            // => si la primera letra es minúscula, capitalizar
            //    sino, dejar como está.
            return caracterPrevio
                 + (minuscInicial ? minuscInicial.toLocaleUpperCase(locale) : mayuscInicial);
        }
    );
}

//Evento para mostrar el resultado cada vez que se ingresa un caracter
document.getElementById('ingreso')
    .addEventListener('input',
        event => {
            document.getElementById('resultado')
                .innerText = titleCase(event.target.value);
        }
    );

<input type="text"
       id="ingreso"
       placeholder="Escribí texto para llevar a mayúsculas la primera letra de cada palabra"
       style="width:100%">
<pre id="resultado">

Seeing the answers given, I'll give you a solution without using regex .

Characteristics of the function:

• You can pass a single word or a whole string
• Respect tildes and umlauts
• We do not use regex

function capitalizarPalabras( val ) {
  
  return val.toLowerCase()
            .trim()
            .split(' ')
            .map( v => v[0].toUpperCase() + v.substr(1) )
            .join(' ');  
}

console.log( capitalizarPalabras( 'hola' ) );
console.log( capitalizarPalabras( ' hola' ) ); // Empieza con un espacio
console.log( capitalizarPalabras( 'hola mundo ' ) );
console.log( capitalizarPalabras( 'ñoño óle' ) );
console.log( capitalizarPalabras( '1 ñoño óle' ) );
console.log( capitalizarPalabras( ' über öll mi au' ) );
console.log( capitalizarPalabras( 'algun texto sin sentido. y este aún menos. ' ) );