A recreation of tabs-to-spaces.html on my formatting-help repository.

_tabs-to-spaces.dart

import "dart:html";

final input = querySelector("#input") as TextAreaElement;
final output = querySelector("#output") as TextAreaElement;
final radio = querySelector("#tabs2spaces") as RadioButtonInputElement;
final button = querySelector("#button") as ButtonElement;
final spacesPerTabs = querySelector("#spacesPerTabs") as InputElement;
// Setup elements

void main() {
    spacesPerTabs.value = "4";
    input.focus();
    // Init
    
    void Function(Event) update = (event) {
        if (radio.checked as bool) {
            // We want tabs to spaces
            output.value = input.value?.replaceAllMapped(
                RegExp(r"^\t*", multiLine: true),
                // Regex matches string-start/newline, then multiple tabs
                (n) => " " * (n[0]!.length * int.tryParse(spacesPerTabs.value ?? "4") ?? 4)
                // Check number of tabs, and multiply that by wanted num of spaces
            );
        } else {
            // We want spaces to tabs
            output.value = input.value?.replaceAllMapped(
                RegExp("^ *", multiLine: true), // Newline/string-start, then spaces
                (n) => "\t"*(n[0]!.length/int.parse(spacesPerTabs.value ?? "4")).floor()
                // Divide that by spaces per tab, and use that many tabs
            );
        }
    };
    
    input.onInput.listen(update);
    button.onClick.listen(update);
    spacesPerTabs.onInput.listen(update);
    radio.onChange.listen(update);
    (querySelector("#spaces2tabs") as RadioButtonInputElement)
        .onChange.listen(update);
    // Add event listeners to all the elements that receive input
}

index.html

<h1>Replacing tabs with spaces, or vice versa</h1>
<h2>Do you ever get frustrated when someone uses tabs and you don't like tabs,
<br>or someone insists on using spaces while you prefer tabs?</h2>
<p>Then this is for you!</p>
<textarea id="input" placeholder="Input here..."></textarea>
<br>
<button id="button">Switch!</button>
<div>
    <input type="radio" name="tabsorspaces" value="tabs2spaces" id="tabs2spaces" checked>
    <label for="tabs2spaces">Tabs to spaces</label>
</div>
<div>
    <input type="radio" name="tabsorspaces" value="spaces2tabs" id="spaces2tabs">
    <label for="spaces2tabs">Spaces to tabs</label>
</div>
Spaces per tab:
<input type="number" id="spacesPerTabs">
<br>
<textarea id="output" placeholder="Output here."></textarea>
<style>
    html {
        font-family: sans-serif;
        tab-size: 4;
        -moz-tab-size: 4;
        -webkit-tab-size: 4;
        margin: 10px;
        color: white;
    }
    textarea {
        width: 500px;
        height: 200px;
    }
    h1, h2 {
        font-weight: 400;
    }
</style>

https://dartpad.dev/aec0c9059054c13e64ca4cdc2a587be8?null_safety=true

Made with ❤️ by Dartpad, the official online IDE/code runner for Dart, which allows even HTML to be used.