Sharepoint 2010 - Modify Documents Properties with ECMA Script

Finally I found the way to modify document properties with ECMA Script!
It's not an hard work for Document Libraries with no mandatory check out.

Here the script:

function updateDocument(action, docLibId, docId, fieldName, fieldValue) {
    var context = new SP.ClientContext.get_current();  
    var web = context.get_web();  
    var docLibGuid = new SP.Guid(docLibId);  
    var docLib = web.get_lists().getById(docLibGuid);  
    var doc = docLib.getItemById(docId);  
    doc.set_item(fieldName, fieldValue);    
    doc.update();  
    context.executeQueryAsync(Function.createDelegate(this, this.successUpdateDocument), Function.createDelegate(this, this.failed)); 
}
function successUpdateDocument(sender, args) {  
    //reload page if success  
    location.reload(true);
}

But when we set check out mandatory, we have message that we cannot modify document not checked out.
So, I have to check out document in this script.
I change script as shown below:

function updateDocument(action, docLibId, docId, fieldName, fieldValue) {
    var context = new SP.ClientContext.get_current();  
    var web = context.get_web();  
    var docLibGuid = new SP.Guid(docLibId);  
    var docLib = web.get_lists().getById(docLibGuid);  
    var doc = docLib.getItemById(docId);  
 
    //perform check out  
    var file = doc.get_file();  
    context.load(file);  
    file.checkOut();  
 
    //set values  
    doc.set_item(fieldName, fieldValue);    
    doc.update();  
 
    //then check in document    
    file.checkIn();    
    context.executeQueryAsync(Function.createDelegate(this, this.successUpdateDocument), Function.createDelegate(this, this.failed));
}

And it works fine!