A recreation of trim-whitespace.html on my formatting-help repository.

_trim-whitespace.dart

import "dart:html";

final input = querySelector("#input") as TextAreaElement;
final target = querySelector("#output") as TextAreaElement;
final number = querySelector("#amount") as InputElement;
final trigger = querySelector("#trigger") as ButtonElement;
// Setup elements

void main() {
    input.addEventListener("input", (Event event) {
        number.value = RegExp(r"^\s*")
            .firstMatch(input.value?.split("\n")[0] as String)?[0]
            ?.length
            .toString(); // Guess the amount of spaces we want
    });
    
    input.focus(); // Focus on the textarea
    number.value = "0"; // Initialize it

    trigger.addEventListener("click", (Event event) {
        final StringBuffer temp = new StringBuffer(); // We will expand on this
        final int trimNumber;
        try {
            trimNumber = int.parse(number.value as String);
        } on FormatException {
            return; // If we reach here, the user has likely entered an invalid number
        }
        for (String currentLine in input.value!.split("\n")) {
            if (currentLine != "") {
                temp.write(currentLine.replaceFirst(
                    RegExp(r"^" + r"\s" * trimNumber), ""
                    // Match string-start, then designated number of spaces/tabs
                ));
            }
            temp.write("\n"); // Add a newline so it's not all mashed together
        }
        target.value = temp.toString(); 
        // We have the result we want, then update the output textarea
    });
}

index.html

<style>
    textarea {
        width: 500px;
        height: 200px;
    }


    html {
        color: white;
        font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", "Google Sans", Roboto, Ubuntu, sans-serif;
        tab-size: 4;
        -moz-tab-size: 4;
        -webkit-tab-size: 4;
    }
</style>
<h1>Remove indentation wacky whitespace</h1>
<h2>When your JS or CSS is indented along with the HTML, it will show with awkward whitespace to the left of it.</h2>
<p>Paste it here to remove that.</p>
<textarea id="input"></textarea>
<br>
Trim <input type="number" id="amount"> off the beginning of each line
<br>
<button id="trigger">Trim!</button>
<br>
<textarea id="output"></textarea>

https://dartpad.dev/eb896fef2a75cf4b1bb182aed65b79ae?null_safety=true

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