How to detect if I'm running in AWS Lambda environment?

There is a process.env property that you can check:

const isLambda = !!process.env.LAMBDA_TASK_ROOT;

if (isLambda) {
  // You're on AWS Lambda
} else {
  // Local or elsewhere
}

Credit to watson/is-lambda for the discovery.

Edit: Official AWS source (with more env vars) https://docs.aws.amazon.com/lambda/latest/dg/configuration-envvars.html

The best way to handle this is to track it yourself. In particular, you can set a global variable or environment variable after the lambda entry point. For example, in node.js:

exports.handler = function(event, context, callback) {
    process.env['LAMBDA_ENV'] = 'true';
    ...
};

If you want to detect whether or not your code is running in local Lambda (aws-sam-cli) or real Lambda, there's the AWS_SAM_LOCAL environment variable.

function isRunningLocalLambda() {
    return process.env.AWS_SAM_LOCAL === 'true';
}