Hello 
it works for me. 

var module_name = "FileOpenJS";
var module_ver = "1.0";
var module_title = "FileOpenJS";


function openFile() {
    var obj = newEditor();
    var filename = 'testfile';
    obj.openFile('I:\' + filename + '.txt');
}

function Init() {
    addMenuItem("Open file test", "", "openFile", "");
}

-- 
<https://forum.pspad.com/read.php?2,72730,72757>
PSPad freeware editor https://www.pspad.com

Odpovedet emailem