Add line numbers to text area using javascript

Issue

I wanted to add line numbering to a textarea.

I get the number of lines by using

textAreaElement.value.split("\n").length;

and maintain a div with span elements that use the count as content on before to add line numbers.

.line-number {
    width: 100%;
    display: block;
    text-align: right;
    line-height:1.5em;
    border-bottom: thin;
    font-family:'CascadiaCode Nerd Font', monospace;
    font-size: 2rem;
    color: #fff;
    opacity: 0.8;
    padding: 0 0.4em;
}
.line-number::before {
    counter-increment: line;
    content: counter(line);
    font-size: 1em;
    user-select: none;
}

However this has a fixed height, I want to implement this for a text area with word wrap (no horizontal scrollbar), where every line can technically have multiple lines terminated by a "\n".

My line of thought was to prepare an array of heights for each lines but I have no idea how to get the height of each separate line.

update: I switched to an editable div, but i want my line number to be of the same height as its corresponding div.

const   lineEnum    =   {
    state: false,
    count:  0,
    gutter: document.getElementsByClassName("line-numbers")[0],
    update: (box)   => {
        let delta   =   box.children.length -   lineEnum.count;
        if (box.children.length ==  0)  delta++;
        
        console.log({
            delta:  delta,
            count:  lineEnum.count,
            length: box.children.length,
        });
        if (delta   >   0   &&  lineEnum.state) {
            const   frag = document.createDocumentFragment();
            while   (delta  >   0) {
                const   line_number =   document.createElement("span");
                line_number.className   =   "line-num";
                frag.appendChild(line_number);
                lineEnum.count++;
                delta--;
            }
            
            lineEnum.gutter.appendChild(frag);
        }   else {
            if (lineEnum.count  +   delta   === 0) delta++;
            while   (delta < 0 && lineEnum.gutter.lastChild) {
                lineEnum.gutter.removeChild(lineEnum.gutter.lastChild);
                lineEnum.count--;
                delta++;
            }
        }
    },
    init:   (box)   => {
        if (lineEnum.state) return;
        lineEnum.state = true;
        lineEnum.update(box);
    },
    remove: (box)   => {
        if  (!lineEnum.state || !lineEnum.gutter.firstChild) return;
        lineEnum.gutter.innerHtml   =   "";
        lineEnum.state  =   false;
    },
};

const   callback = (mutationList,   observer)   => {
    let mutation =  mutationList[mutationList.length - 1];
    if  (mutation.type === "childList") {
        console.log(mutation);
        lineEnum.update(mutation.target);
    }
};

const   observer = new MutationObserver(callback);
const   config = { childList:   true };

const   editor = document.getElementsByClassName("code-input")[0];
observer.observe(editor, config);

lineEnum.init(editor);

.window-body{
    position: fixed;
    height: 100%;
    top: 25px;
    width: 100%;
    display: flex;
}

.line-numbers {
    width: 5em;
    padding: 0;
    height: 100%;
    word-break: break-all;
    overflow: hidden;
    display: inline-block;
    counter-reset: line;
    background-color: gray;
    opacity: 0.8;
}

.line-num {
    width: 100%;
    display: block;
    text-align: middle;
    line-height:1.5em;
    border-bottom: thin;
    font-family:'Arial', monospace;
    font-size: 2rem;
    color: #fff;
    opacity: 0.8;
    padding: 0 1em;
}
.line-num::before {
    counter-increment: line;
    content: counter(line);
    font-size: 1em;
    user-select: none;
}

.code-input{
    margin: 0;
    border: 0;
    padding: 0;
    outline: 0;
    list-style: none;

    display: inline-block;
    flex-grow: 1;
    height: 100%;
    word-break: break-all;
    overflow: hidden;

    border:none;
    font-family:'Arial', monospace;
    font-size:2rem;
    background: white;
    white-space:pre-wrap;
    line-height:1.5em;
    word-wrap: break-word;
    resize:none;
}

<div class="window-body">
        <div class="line-numbers"></div>
        <div class="code-input" contenteditable="true"></div>
</div>

Solution

Man I wasted so much time on using javascript when css magic would just have done the trick.

Here is how I did it,

body {
    background-color: #000;
    height: 100vh;
    width: 100vw;
    margin: 0px;
}

.editor-wrapper {
  height: 100vh;
  width: 100vw;
  overflow-y: auto;
    counter-reset: line;
}
.editor{
    margin: 0;
    border: 0;
    padding: 0;
    outline: 0;
    list-style: none;

    height: 100%;
    width: 100%;
    word-wrap: break-word;
    word-break: break-all;

    font-size:2rem;
    line-height: 1.5em;
    font-feature-settings: common-ligatures; 
    -ms-font-feature-settings: common-ligatures;
    color:rgba(255, 255, 255, 0.7);
    resize:none;

}

.editor div {
    padding-left: 5rem;
    position: relative;
}

.editor div::before {
    counter-increment: line;
    content: counter(line);
    font-size: 1em;
    user-select: none;
    width: 5rem;
    text-align: right;
    left: 0;

    position: absolute;
}

<div class="editor-wrapper">
    <div class="editor" contenteditable="true">
        <div></div>
    </div>
</div>

Answered By - Blaine