SetCaretAtEnd function accepts a javascript dom object and sets cursor to end.

setTextboxCursorAtEnd.js

function SetCaretAtEnd(elem) {
	var elemLen = elem.value.length;
	// For IE Only
	if (document.selection) {
		// Set focus
		elem.focus();
		// Use IE Ranges
		var oSel = document.selection.createRange();
		// Reset position to 0 & then set at end
		oSel.moveStart('character', -elemLen);
		oSel.moveStart('character', elemLen);
		oSel.moveEnd('character', 0);
		oSel.select();
	}
	else if (elem.selectionStart || elem.selectionStart == '0') {
		// Firefox/Chrome
		elem.selectionStart = elemLen;
		elem.selectionEnd = elemLen;
		elem.focus();
	} // if
};

SetCaretAtEnd($('.m-status-item .editPostInput')[0]); // This is how a jQuery object is passes. note [0].