Code indent function for <textarea onkeydown="tryIndent(event)">

indent.js

function tryIndent(event) {
	if (event.code != "Tab") {
		return;
	}
	event.preventDefault();
	var text = event.target;
	var start = Math.max(text.value.lastIndexOf("\n", text.selectionStart - 1), 0);
	var end = text.selectionEnd;
	var str = text.value.slice(start, end);
	var origLength = str.length;
	if (event.shiftKey) {
		str = (str.startsWith(" ") ? str.slice(1) : str).replace(/\n /g, "\n");
	} else {
		str = (start == 0 ? " " + str : str).replace(/\n/g, "\n ");
	}
	text.value = text.value.slice(0, start) + str + text.value.slice(end);
	text.selectionStart = (start == 0) ? 0 : start + 1;
	text.selectionEnd = end + str.length - origLength;
}