Get root site url using CSOM

var clientContext = new SP.ClientContext();
var owebsite = clientContext.get_site.get_rootWeb();

Without client object model you can use the following

var siteCollectionPath= _spPageContextInfo.siteServerRelativeUrl;

SharePoint  Field validation for special characters.

Friday, October 25, 2013

8:42 PM

can be used in single field or entire list validation

=AND(IF(ISERROR(FIND(",",Title)),TRUE),IF(ISERROR(FIND("&",Title)),TRUE),IF(ISERROR(FIND("!",Title)),TRUE),IF(ISERROR(FIND("@",Title)),TRUE),IF(ISERROR(FIND("~",Title)),TRUE),IF(ISERROR(FIND("#",Title)),TRUE),IF(ISERROR(FIND("$",Title)),TRUE),IF(ISERROR(FIND("%",Title)),TRUE),IF(ISERROR(FIND("^",Title)),TRUE),IF(ISERROR(FIND("*",Title)),TRUE),IF(ISERROR(FIND("(",Title)),TRUE),IF(ISERROR(FIND(")",Title)),TRUE),IF(ISERROR(FIND("-",Title)),TRUE),IF(ISERROR(FIND("=",Title)),TRUE),IF(ISERROR(FIND("+",Title)),TRUE),IF(ISERROR(FIND(":",Title)),TRUE),IF(ISERROR(FIND(";",Title)),TRUE),IF(ISERROR(FIND("<",Title)),TRUE),IF(ISERROR(FIND(">",Title)),TRUE),IF(ISERROR(FIND("?",Title)),TRUE),IF(ISERROR(FIND("'",Title)),TRUE),IF(ISERROR(FIND("{",Title)),TRUE),IF(ISERROR(FIND("}",Title)),TRUE),IF(ISERROR(FIND("[",Title)),TRUE),IF(ISERROR(FIND("]",Title)),TRUE),IF(ISERROR(FIND(".",Title)),TRUE),IF(ISERROR(FIND("/",Title)),TRUE),IF(ISERROR(FIND("\",Title)),TRUE),IF(ISERROR(FIND("""",Title)),TRUE))

Check current user permissions using jsom

function checkifUserHasEditPermissions() {
context = new SP.ClientContext.get_current();
web = context.get_web();
this._currentUser = web.get_currentUser();
context.load(this._currentUser);
context.load(web,'EffectiveBasePermissions');
context.executeQueryAsync(Function.createDelegate(this, this.onSuccessMethod), Function.createDelegate(this, this.onFailureMethod));
}
function onSuccessMethod(sender, args) 
{
if (web.get_effectiveBasePermissions().has(SP.PermissionKind.editListItems)) 
{
//User Has Edit Permissions
}
}}

Check if file exist using script.

1)
same domain this should work:
function UrlExists(url)
{
    var http = new XMLHttpRequest();
    http.open('HEAD', url, false);
    http.send();
    return http.status!=404;
}


or

function fileExists(url) {
    if(url){
        var req = new XMLHttpRequest();
        req.open('GET', url, false);
        req.send();
        return req.status==200;
    } else {
        return false;
    }
}

2)
function ImageExist(url)
{
   var img = new Image();
   img.src = url;
   return img.height != 0;
}

3)

<img src="image.gif" onerror="imgError()" />

4)

$.get(url)
    .done(function() { 
        // exists code 
    }).fail(function() { 
        // not exists code
    })

5)

$.ajax({
    url:'http://www.example.com/somefile.ext',
    type:'HEAD',
    error: function()
    {
        //file not exists
    },
    success: function()
    {
        //file exists
    }
});

For a client computer this can be achieved by:

try
{
  var myObject, f;
  myObject = new ActiveXObject("Scripting.FileSystemObject");
  f =   myObject.GetFile("C:\\img.txt");
  f.Move("E:\\jarvis\\Images\\");
}
catch(err)
{
  alert("file does not exist")
}

6)Image exists. 

function checkImage(src) {
  var img = new Image();
  img.onload = function() {
      // code to set the src on success
      $('#image-test').css('background', 'url(' + src + ') no-repeat 50% 50%');
  };
  img.onerror = function() {
    // doesn't exist or error loading
    alert('no image');
  };

  img.src = src; // fires off loading of image
}

checkImage('http://www.google.com/images/logo_sm.gif');