docs(main): adding info about remove undefined values for doc client …

UPGRADING.md

@@ -458,13 +458,17 @@ S3 client in v3 supports S3 global client, or following region redirects, if an
 
 ## DynamoDB Document Client
 
-In v2, you can use the [`AWS.DynamoDB.DocumentClient` class](https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/DynamoDB/DocumentClient.html)
-to call DynamoDB API with native JavaScript types like Buffer, Array, and Object. It thus simplifies working with items
-in Amazon DynamoDB by abstracting away the notion of attribute values.
+### Basic Usage of DynamoDB Document Client in v3
 
-In v3, equivalent [`@aws-sdk/lib-dynamodb`](https://docs.aws.amazon.com/AWSJavaScriptSDK/v3/latest/modules/_aws_sdk_lib_dynamodb.html)
-is available. It's similar to normal service clients from v3 SDK, with the difference that it takes a basic DynamoDB
-client in its constructor. Here's an brief example:
+- In v2, you can use the [`AWS.DynamoDB.DocumentClient` class](https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/DynamoDB/DocumentClient.html)
+  to call DynamoDB API with native JavaScript types like Buffer, Array, and Object. It thus simplifies working with items
+  in Amazon DynamoDB by abstracting away the notion of attribute values.
+
+- In v3, equivalent [`@aws-sdk/lib-dynamodb`](https://docs.aws.amazon.com/AWSJavaScriptSDK/v3/latest/modules/_aws_sdk_lib_dynamodb.html)
+  is available. It's similar to normal service clients from v3 SDK, with the difference that it takes a basic DynamoDB
+  client in its constructor.
+
+Example:
 
 ```javascript
 import { DynamoDBClient } from "@aws-sdk/client-dynamodb"; // ES6 import
@@ -489,6 +493,39 @@ await ddbDocClient.send(
 );
 ```
 
+### Undefined Values in Marshalling
+
+- In v2 `undefined` values in objects were automatically omitted during the marshalling process to DynamoDB.
+
+- In v3, using [`@aws-sdk/lib-dynamodb`](https://docs.aws.amazon.com/AWSJavaScriptSDK/v3/latest/modules/_aws_sdk_lib_dynamodb.html), the default behavior has changed: `undefined` values in objects are now included during marshalling. Developers need to explicitly manage undefined values to exclude them using the `removeUndefinedValues` flag.
+
+Example:
+
+```js
+import { DynamoDBClient } from "@aws-sdk/client-dynamodb";
+import { DynamoDBDocumentClient, PutCommand } from "@aws-sdk/lib-dynamodb";
+import { marshall } from "@aws-sdk/util-dynamodb";
+
+const client = new DynamoDBClient({});
+
+const ddbDocClient = DynamoDBDocumentClient.from(client);
+
+const item = {
+  id: "123",
+  content: undefined,
+};
+
+const marshalledItem = marshall(item, { removeUndefinedValues: true });
+
+// procceed with making the API call as usual:
+await ddbDocClient.send(
+  new PutCommand({
+    TableName: "YourTableName",
+    Item: marshalledItem,
+  })
+);
+```
+
 More examples and configurations are available in the [package README](https://github.com/aws/aws-sdk-js-v3/blob/main/lib/lib-dynamodb/README.md).
 
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