# @title Making Requests in the Browser

# Making Requests in the Browser

A "request" to an AWS service includes the full request and response lifecycle
of a call to an operation on a service object, including any retries that are
transparently attempted on your behalf. A request is encapsulated in the SDK by
the `AWS.Request` object. The semantics of a request are described below,
specifically, the support for callbacks, events, and streaming of raw HTTP
response data.

## Asynchronous Callbacks

All requests made through the SDK are asynchronous and use a
callback interface. Each service method that kicks off a request
can accept a callback as the last parameter with the signature
`function(error, data) { ... }`. This callback will be called when
the response or error data is available.

For example, the following service method can be called with
a standard callback to retrieve the response data or error:

```javascript
new AWS.S3().listObjects({Bucket: 'myBucket'}, function(error, data) {
  if (error) {
    console.log(error); // an error occurred
  } else {
    console.log(data); // request succeeded
  }
});
```

The `error` and `data` parameters are described in the "Response Object"
section below.

Note that if you do not specify a callback, the operation will
return an `AWS.Request` object that must be manually sent using
the `send()` method:

```javascript
// create the AWS.Request object
var request = new AWS.S3().listObjects({Bucket: 'myBucket'});

// register a callback to report on the data
request.on('success', function(resp) {
  console.log(resp.data); // log the successful data response
});

// send the request
request.send();
```

### The Response Object (`AWS.Response`)

The response object is passed into each callback function so
that you can access response data. The `AWS.Response` object that
is passed in contains two important properties to get at this data:

When using the standard callback mechanism, the two properties will
be made available as parameters on the callback method in the form:
`function(error, data) { ... }`

#### The `data` property

The `response.data` property contains the serialized object data
retrieved from the service request. For instance, for an
Amazon DynamoDB `listTables` method call, the response data might 
look like this:

```javascript
> response.data
{ TableNames: 
   [ 'table1', 'table2', ... ] }
```

The `data` property can be null if an error occurs (see below).

#### The `error` property

In the event of a service error (or transfer error), the
`response.error` property will be filled with the given
error data in the form:

```javascript
{ code: 'SHORT_UNIQUE_ERROR_CODE',
  message: 'Some human readable error message' }
```

In the case of an error, the `data` property will be null.
Note that if you handle events that can be in a failure state,
you should always check whether `response.error` is set
before attempting to access the `response.data` property.

#### The `request` property

Access to the originating request object is available through this
property. For example, to access the parameters that were sent
with a request:

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

### Simplified Callback Method

Each operation supports a simplified callback that can be passed as the last
parameter to any service operation. The callback function should
accept an `error` parameter, followed by the `data` from the response.

For example:

```javascript
s3.listBuckets(function(error, data) {
  if (err) {
    console.log(error); // error is Response.error
  } else {
    console.log(data); // data is Response.data
  }
});
```

Prints (assuming the request succeeded):

```javascript
{ Owner: { ID: '...', DisplayName: '...' },
  Buckets:
   [ { Name: 'someBucketName', CreationDate: someCreationDate },
     { Name: 'otherBucketName', CreationDate: otherCreationDate } ],
  RequestId: '...' }
```

The error and data parameters accepted are equivalent to the `error` and
`data` properties discussed in the `AWS.Response` response object section
above.

If you are passing parameters to the operation, the callback should be placed
after the parameters:

```
s3.getObject({Bucket: 'bucket', Key: 'key'}, function(err, data) {
  // ...
});
```

### AWS.Request Events

You can alternatively register callbacks on events provided by the
`AWS.Request` object returned by each service operation method.
This request object exposes the `success`, `error`, `complete`, and `httpData`
events, each taking a callback that accepts the response object.

Note that if you omit the simplified callback parameter on the operation
method, you must call `send()` on the returned request object in order to
kick off the request to the remote server.

#### Event: 'success'

```javascript
req.on('success', function(response) { ... });
```

This event triggers when a successful response
from the server is returned. The response contains a `.data` field
with the serialized response data from the service.

For example:

```javascript
s3.listBuckets().done(function(response) {
  console.log(response.data);
}).send();
```

Prints:

```javascript
{ Owner: { ID: '...', DisplayName: '...' },
  Buckets: 
   [ { Name: 'someBucketName', CreationDate: someCreationDate },
     { Name: 'otherBucketName', CreationDate: otherCreationDate } ],
  RequestId: '...' }
```

#### Event: 'error'

```javascript
req.on('error', function(error, response) { ... });
```

The `error` event works similarly to the `success` event, except that it
triggers in the case of a request failure. In this case, `response.data`
will be `null` and the `response.error` field will be filled with
the error data. Note that the `error` object is also passed directly
as the first parameter to the event:

```javascript
s3.config.credentials.accessKeyId = 'invalid';
s3.listBuckets().fail(function(error, response) {
  console.log(error);
  // or:
  console.log(response.error);
}).send();
```

Prints:

```javascript
{ code: 'Forbidden', message: null }
```

#### Event: 'complete'

```javascript
req.on('complete', function(response) { ... });
```

The `complete` event triggers a callback in any final state of a request, i.e.,
both `success` and `error`. Use this callback to handle any request cleanup
that must be executed regardless of the success state. Note that if you
do intend to use response data inside of this callback, you must check
for the presence of `response.data` or `response.error` before attempting
to access either property. For example:

```javascript
request.on('complete', function(response) {
  if (response.error) {
    // an error occurred, handle it
  } else {
    // we can use response.data here
  }
}).send();
```

#### Event: 'httpData'

```javascript
req.on('httpData', function(chunk, response) { ... });
```

<p class="note">If you register a <code>httpData</code> callback,
  <code>response.data</code> will still contain serialized output
  for the entire request. It will be your responsibility to remove
  the default 'httpData' listener if you do not wish to have the
  extra parsing and memory overhead from the built-in handlers.
</p>

The `httpData` event is used to stream response data from the
service packet-by-packet. This event is mostly used for large responses,
when it is inefficient (or impossible) to load the entire response into
memory.

Note that this event contains an extra `chunk` parameter containing the
actual data passed on by the server.

## Multiple Callbacks and Chaining

You can register multiple callbacks on any request object. The
callbacks can be registered for different events, or all for the
same event. In addition, you can chain callback registration, for
example:

```javascript
request.
  on('success', function(response) {
    console.log("Success!");
  }).
  on('error', function(response) {
    console.log("Error!");
  }).
  on('complete', function() {
    console.log("Always!");
  }).
  send();
```

The above example will print either "Success! Always!", or "Error! Always!",
depending on whether the request succeeded or not.