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Using the Response Object

After a service object method has been called, it returns an AWS.Response object by passing it to your callback function. You access the contents of the response through the properties of the AWS.Response object. There are two properties of the AWS.Response object you use to access the contents of the response:

When using the standard callback mechanism, these two properties are provided as parameters on the anonymous callback function as shown in the following example.

function(error, data) {
    if (error) {
        // error handling code
        console.log(error);
    } else {
        // data handling code
        console.log(data);
    }
}

Accessing Data Returned in the Response Object

The data property of the AWS.Response object contains the serialized data returned by the service request. When the request is successful, the data property contains an object that contains a map to the data returned. The data property can be null if an error occurs.

Here is an example of calling the getItem method of a DynamoDB table to retrieve the file name of an image file to use as part of a game.

// Initialize parameters needed to call DynamoDB
var slotParams = {
    Key : {'slotPosition' : {N: '0'}},
    TableName : 'slotWheels',
    ProjectionExpression: 'imageFile'
};

// prepare request object for call to DynamoDB
var request = new AWS.DynamoDB({region: 'us-west-2', apiVersion: '2012-08-10'}).getItem(slotParams);
// log the name of the image file to load in the slot machine
request.on('success', function(response) {
    // logs a value like "cherries.jpg" returned from DynamoDB
    console.log(response.data.Item.imageFile.S);
});
// submit DynamoDB request
request.send();

For this example, the DynamoDB table is a lookup of images that show the results of a slot machine pull as specified by the parameters in slotParams.

Upon a successful call of the getItem method, the data property of the AWS.Response object contains an Item object returned by DynamoDB. The returned data is accessed according to the request's ProjectionExpression parameter, which in this case means the imageFile member of the Item object. Because the imageFile member holds a string value, you access the file name of the image itself through the value of the S child member of imageFile.

Paging Through Returned Data

Sometimes the contents of the data property returned by a service request span multiple pages. You can access the next page of data by calling the response.nextPage method. This method sends a new request. The response from the request can be captured either with a callback or with success and error listeners.

You can check to see if the data returned by a service request has additional pages of data by calling the response.hasNextPage method. This method returns a boolean to indicate whether calling response.nextPage returns additional data.

s3.listObjects({Bucket: 'bucket'}).on('success', function handlePage(response) {
    // do something with response.data
    if (response.hasNextPage()) {
        response.nextPage().on('success', handlePage).send();
    }
}).send();

Accessing Error Information from a Response Object

The error property of the AWS.Response object contains the available error data in the event of a service error or transfer error. The error returned takes the following form.

{ code: 'SHORT_UNIQUE_ERROR_CODE', message: 'a descriptive error message' }

In the case of an error, the value of the data property is null. If you handle events that can be in a failure state, always check whether the error property was set before attempting to access the value of the data property.

Accessing the Originating Request Object

The request property provides access to the originating AWS.Request object. It can be useful to refer to the original AWS.Request object to access the original parameters it sent. In the following example, the request property is used to access the Key parameter of the original service request.

s3.getObject({Bucket: 'bucket', Key: 'key'}).on('success', function(response) {
   console.log("Key was", response.request.params.Key);
}).send();