Html

$(function() {
	var enabled = true;
	$("textarea.tabSupport").keydown(function(e) {

		// Escape key toggles tab on/off
		if (e.keyCode==27)
		{
			enabled = !enabled;
			return false;
		}

		// Enter Key?
		if (e.keyCode === 13 && enabled)
		{
			// selection?
			if (this.selectionStart == this.selectionEnd)
			{
				// find start of the current line
				var sel = this.selectionStart;
				var text = $(this).val();
				while (sel > 0 && text[sel-1] != '\n')
				sel--;

				var lineStart = sel;
				while (text[sel] == ' ' || text[sel]=='\t')
				sel++;

				if (sel > lineStart)
				{
					// Insert carriage return and indented text
					document.execCommand('insertText', false, "\n" + text.substr(lineStart, sel-lineStart));

					// Scroll caret visible
					this.blur();
					this.focus();
					return false;
				}
			}
		}

		// Tab key?
		if(e.keyCode === 9 && enabled)
		{
			// selection?
			if (this.selectionStart == this.selectionEnd)
			{
				// These single character operations are undoable
				if (!e.shiftKey)
				{
					document.execCommand('insertText', false, "\t");
				}
				else
				{
					var text = this.value;
					if (this.selectionStart > 0 && text[this.selectionStart-1]=='\t')
					{
						document.execCommand('delete');
					}
				}
			}
			else
			{
				// Block indent/unindent trashes undo stack.
				// Select whole lines
				var selStart = this.selectionStart;
				var selEnd = this.selectionEnd;
				var text = $(this).val();
				while (selStart > 0 && text[selStart-1] != '\n')
					selStart--;
				while (selEnd > 0 && text[selEnd-1]!='\n' && selEnd < text.length)
					selEnd++;

				// Get selected text
				var lines = text.substr(selStart, selEnd - selStart).split('\n');

				// Insert tabs
				for (var i=0; i<lines.length; i++)
				{
					// Don't indent last line if cursor at start of line
					if (i==lines.length-1 && lines[i].length==0)
						continue;

					// Tab or Shift+Tab?
					if (e.shiftKey)
					{
						if (lines[i].startsWith('\t'))
							lines[i] = lines[i].substr(1);
						else if (lines[i].startsWith("    "))
							lines[i] = lines[i].substr(4);
					}
					else
						lines[i] = "\t" + lines[i];
				}
				lines = lines.join('\n');

				// Update the text area
				this.value = text.substr(0, selStart) + lines + text.substr(selEnd);
				this.selectionStart = selStart;
				this.selectionEnd = selStart + lines.length;
			}

			return false;
		}

		enabled = true;
		return true;
	});
});

textarea
{
  width: 100%;
  height: 100px;
  tab-size: 4;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<textarea class="tabSupport">if (something)
{
	// This textarea has "tabSupport" CSS style
	// Try using tab key
	// Try selecting multiple lines and using tab and shift+tab
	// Try pressing enter at end of this line for auto indent
	// Use Escape key to toggle tab support on/off
	//     eg: press Escape then Tab to go to next field
}
</textarea>
<textarea>This text area doesn't have tabSupport class so disabled here</textarea>