Separate the Lambda handler from your core logic. This allows you to make a more unit-testable function. In Node.js this may look like:

Take advantage of execution environment reuse to improve the performance of your function. Initialize

To avoid potential data leaks across invocations, don’t use the execution environment to store user data, events, or other information with security implications. 

Use a keep-alive directive to maintain persistent connections. Lambda purges idle connections over time. 

Use environment variables to pass operational parameters to your function. For example, if you are writing to an Amazon S3 bucket, instead of hard-coding the bucket name you are writing to, configure the bucket name as an environment variable.

Control the dependencies in your function's deployment package. The AWS Lambda execution environment contains a number of libraries such as the AWS SDK for the Node.js and Python runtimes (a full list can be found here: Lambda runtimes).

Minimize your deployment package size to its runtime necessities. This will reduce the amount of time that it takes for your deployment package to be downloaded and unpacked ahead of invocation.

Reduce the time it takes Lambda to unpack deployment packages authored in Java by putting your dependency .jar files in a separate /lib directory. This is faster than putting all your function’s code in a single jar with a large number of .class files. See Deploy Java Lambda functions with .zip or JAR file archives for instructions.

Minimize the complexity of your dependencies. Prefer simpler frameworks that load quickly on execution environment startup. For example, prefer simpler Java dependency injection (IoC) frameworks like Dagger or Guice, over more complex ones like Spring Framework.

Avoid using recursive code in your Lambda function, wherein the function automatically calls itself until some arbitrary criteria is met. This could lead to unintended volume of function invocations and escalated costs. If you do accidentally do so, set the function reserved concurrency to 0 immediately to throttle all invocations to the function, while you update the code.

Do not use non-documented, non-public APIs in your Lambda function code. For AWS Lambda managed runtimes, Lambda periodically applies security and functional updates to Lambda's internal APIs. These internal API updates may be backwards-incompatible, leading to unintended consequences such as invocation failures if your function has a dependency on these non-public APIs. See the API reference for a list of publicly available APIs.

Write idempotent code. Writing idempotent code for your functions ensures that duplicate events are handled the same way. Your code should properly validate events and gracefully handle duplicate events. For more information, see How do I make my Lambda function idempotent?. 

https://docs.aws.amazon.com/lambda/latest/dg/best-practices.html