Replace all characters with "_" except the 1° character of each word

Question

I want to replace a sentence every character by _ except the 1° character of each word In Javascript.

Q____ S_________ U__ F____ T____ O_ C_______ P__

This would be the idea to use in large texts. I want to create a function within this code in Javascript:

function maiuscula(id){

    var letra=document.getElementById(id).value;
    letra=letra.split("");
    var tmp="";
        var Word="_";

    for(i=0;i<letra.length;i++){


    if(letra[i-1])
        {
          if(letra[i-1]==" ")
            {
                letra[i]=letra[i].replace(letra[i],letra[i].toUpperCase());
            }
        }

    else
    {
        letra[i]=letra[i].replace(letra[i],letra[i].toUpperCase());
    }

    tmp+=letra[i];

     }

    document.getElementById(id).value=tmp;

To be used in an input:

<input type="text" id="billing:firstname" onkeyup="maiuscula('billing:firstname')" name="billing[firstname]" value="" title="Nome" class="t1 required-entry"/>

Answer 1

Another alternative is to use the String.replace together with the regular expression /\B\w/g:

var valor = 'stack overflow em portugues';

var substituto = valor.replace(/\B\w/g, '_');
// s____ o_______ e_ p________

Where \B is the opposite of \b, \b is limited to corresponding to an exact pattern or word, depending on the position in which it is inserted, for example the expression \b\w corresponds to f and b in foo bar, the other way around \w\b corresponds to o and r.

The expression \B\w corresponds to oo and ar and \w\B to fo and ba. In this other question you have more details about: What good is a Boundary (\b) in a regular expression?

The modifier g indicates that a global search should be made, not return the first time the pattern is matched.

To display the remaining uppercase letters, use the string.toUpperCase:

var valor = 'stack overflow em portugues';
var substituto = valor.replace(/\B\w/g, '_').toUpperCase();

console.log(substituto);
// S____ O_______ E_ P________

Answer 2

The following logic will solve your problem:

var text = "QUOTE";

 function replaceString(text){
    var textRefact;
    for(var i = 0; i < text.length; i++){
       if(i == 0){
       textRefact = text.charAt(i);
    }else{
       textRefact += "_";
    }
 }
   return textRefact;
 }

var newText = replaceString(text);

console.log(newText);