What are event and context in function call in AWS Lambda?

Question

In the following code, what do the arguments event and context refer to?

module.exports.convertTime = (event, context, callback) => {
  const response = {
    statusCode: statusCode,
    body: JSON.stringify({
      message: `${convertedTime}`
    })
  };

  callback(null, response);
}

Answer 1

Event represents the event or trigger that caused the invocation of the lambda. For example, if your lambda is triggered by an upload to S3 this will contain information about the object being uploaded for example:

    {
      "Records": [
        {
          "eventVersion": "2.0",
          "eventTime": "1970-01-01T00:00:00.000Z",
          "requestParameters": {
            "sourceIPAddress": "127.0.0.1"
          },
          "s3": {
            "configurationId": "testConfigRule",
            "object": {
              "eTag": "0123456789abcdef0123456789abcdef",
              "sequencer": "0A1B2C3D4E5F678901",
              "key": "HappyFace.jpg",
              "size": 1024
            },
            "bucket": {
              "arn": bucketarn,
              "name": "sourcebucket",
              "ownerIdentity": {
                "principalId": "EXAMPLE"
              }
            },
            "s3SchemaVersion": "1.0"
          },
          "responseElements": {
            "x-amz-id-2": "EXAMPLE123/5678abcdefghijklambdaisawesome/mnopqrstuvwxyzABCDEFGH",
            "x-amz-request-id": "EXAMPLE123456789"
          },
          "awsRegion": "us-east-1",
          "eventName": "ObjectCreated:Put",
          "userIdentity": {
            "principalId": "EXAMPLE"
          },
          "eventSource": "aws:s3"
        }
      ]
    }

Here is detailed information about events, with an example.

Context Provides information about the invocation, function, and execution environment of your lambda. So you can use this to check the memory allocation or to retrieve the number of milliseconds left before the execution times out. Here is detailed documentation about context, with an example.

Answer 2

From the docs

When Lambda runs your function, it passes a context object to the handler. This object provides methods and properties that provide information about the invocation, function, and execution environment.

Event (and the args) are described here.

To put it more simply, think of event as an input to a regular function. Context is an extra input supplied by AWS to give you a variety of meta context and such.