Bold/Italic/Underline User Text Input

I have a text input, I use DOM to get the text, I show the text. The problem is I want the user to be able to bold/italic/underline some words or part of the text. I was thinking about letting the user write BOLD/ITALIC/UNDERLINE around the text, search for the index of this words, and apply the changes with javascript. I can already do the first two steps, but i don't know how to apply the changes.
I search for ways to turn the text bold with javascript with document.createElement("B"); but using this method is complicated because it would be just specific words or sentences.

var bold = {}; var bld = 0;
var str = 'string BOLD of BOLD the ITALIC input text ITALIC that i UNDERLINE get UNDERLINE using dom';
var show = document.createElement("P");
//Here I use .includes() and .matchAll() to get an array with all the indexes
var regexp = new RegExp("BOLD","g");
let arraybold = [...str.matchAll(regexp)].map(a => a.index);
//ex: arraybold[0] = 7 and arraybold[1] = 15. 
for(key in arraybold) {
    bld = bld + 1;
}
regexp = new RegExp("ITALIC","g");
let arrayitalic = [...str.matchAll(regexp)].map(a => a.index);
regexp = new RegExp("UNDERLINE","g");
let arrayunderline = [...str.matchAll(regexp)].map(a => a.index);
//Here I'm suppose to create the bold/italic/underline tags on the specific words. 
for(i = 1; i <= (bld/2); i++) {
var bold[i] = document.createElement("B");
bold[i].innerText = str.slice(arraybold[i]+4, arraybold[i+1]);
//str.replace(the part of the text with the bold tag that i create);
}
show.innerText = str;
document.body.appendChild(show);

Ps: I can't use innerHTML because i will use the dom later. I would prefer to do this with vanilla javascript. The expected result:

//<p>string <strong> of </strong> the <i> input text </i> that i <u> get</u> using dom</p>

edit: using let instead of var as suggested by @cpprookie

Solution

Try to split string and fill an array with data which structure looks like { text: 'string', type: 'noStyle/BOLD/ITALIC/UNDERLINE' }, finally generate node based on type field.

First split and traverse whole string. Fill textList according to corresponding keywords(BOLD/ITALIC/UNDERLINE);

var textList = [];
var boldText = '';
var italicText = '';
var underlineText = '';
var markAsBold = false;
var markAsItalic = false;
var markAsUnderline = false;

var str = 'string BOLD of BOLD the ITALIC input text ITALIC that i UNDERLINE get UNDERLINE using dom';

str.split(' ').forEach(s => {
  if (s === 'BOLD') {
     // match left BOLD
     if (!markAsBold) {
        markAsBold = true;
     } else {
        // match right BOLD and clear boldText;
       textList.push({
         text: boldText,
         style: 'BOLD'
       });
       boldText = '';
       markAsBold = false;
     } else if (s === 'ITALIC') {
        // same as 'BOLD' process
     } else if (s === 'UNDERLINE') {
        // same as 'BOLD' process
     } else {
        if (markAsBold) {
            boldText += boldText.length > 0 ? s : (' ' + s);
        } else if (markAsItalic) {
            italicText += italicText.length > 0 ? s : (' ' + s);
        } else if (markAsUnderline) {
            underlineText += underlineText.length > 0 ? s : (' ' + s);
        } else {
            textList.push({
                text: s,
                style: 'noStyle'
            })
        }
     }
  }
})

Create node based on type in textList.

  textList.forEach(item => {
    let node;
    if (item.style === 'noStyle') {
      // create a span node
    } else if (item.style === 'BOLD') {
      // create a bold node
    } else if (item.style === 'ITALIC') {
      // create a italic node
    } else {
      // create a node and underline it with css.
    }
    // Append to parentNode at last.
  })