Update dynamodb data by lambda

It looks like you are trying to update an item by using an Expression, and in this case, your UpdateExpression is incorrect. Both the ExpressionAttributeNames and ExpressionAttributeValues are used for placeholder substitution in your expression.

I think your code would look something like this, if you want to set an attribute for an item:

dynamoDB.updateItem({  
    "TableName" : "exampleTable",
    "Key" : {
        "hashAttributeName" : {
            "S" : "thing_ID"
        }
    },
    "UpdateExpression" : "SET #attrName =:attrValue",
    "ExpressionAttributeNames" : {
        "#attrName" : "SessionID"
    },
    "ExpressionAttributeValues" : {
        ":attrValue" : {
            "S" : "maybe this works"
        }
    }
});

This will update an item that looks like this:

{  
    "Item":{  
        "hashAttributeName":"thing_ID"
    }
}

To this:

{  
    "Item":{  
        "hashAttributeName" : "thing_ID",
        "SessionID" : "maybe this works"
    }
}