SP2010: Unable to update list item with JSOM

Question

I have a list column which I want to update using JSOM on button click. Here's my code -

function updateListItem() {
var clientContext = new SP.ClientContext.get_current();
var oList = clientContext.get_web().get_lists().getByTitle('Topics');
this.oListItem = oList.getItemById(20);
clientContext.load(oListItem);
clientContext.executeQueryAsync(Function.createDelegate(this, this.onQuerySucceeded), Function.createDelegate(this, this.onQueryFailed));
}

function onQuerySucceeded() {
var oldData = oListItem.get_item('ViewCount');
var newData = parseInt(oldData) + 1;
alert(newData);
oListItem.set_item('ViewCount', newData);
oListItem.update();
clientContext.executeQueryAsync(Function.createDelegate(this, this.onItemUpdated), Function.createDelegate(this, this.onQueryFailed));
}

function onItemUpdated() {
   alert('Item has been updated!');
}

function onQueryFailed(sender, args) {
    alert('Request failed.');
}

But, it's not working.

Answer 1

To get the properties of List item you have to query the server by calling ExecuteQueryAsync. So it can be like this:

clientContext.load(
        oListItem,
        'Include(Id, DisplayName, HasUniqueRoleAssignments,ViewCount)'
    );
    clientContext.executeQueryAsync(
        Function.createDelegate(this, this.onQuerySucceeded),
        Function.createDelegate(this, this.onQueryFailed)
    );

Answer 2

You are trying to access field ViewCount before executeQueryAsync.

So your code oListItem.get_item('ViewCount'); will fail in updateListItem function.

'executeQueryAsync'Executes the current pending request asynchronously on the server.

No communication occurs with the remote server until you call ExecuteQuery or ExecuteQueryAsync (in this case 'ExecuteQueryAsync').

So you will not be able to access the value of field ViewCount in updateListItem function. But the value will accessible in success Handler.

And after getting this value in success function, you will have to make another request the list to update the value of that field.