Mijn NS TSL toolkit

This toolkit is a set of Typescript decorators and a handler wrapper that removes lots of boilerplate from vanilla Typescript Lambda (TSL) projects.

When to use?

• When building API functions that respond to AWS API Gateway calls, as this library imitates Spring in many ways.

When not to use?

• When building vanilla functions that are directly invoked, since handler responses and thrown errors are automatically serialized to an appropriate HTTP response, which is not needed in such cases.

Features

Error handling

The below example will throw an error from the handler function, which will be caught by the errorHandler function, and turned into a 400 response entity.

function errorHandler(on: ErrorCatcher) {

  on(err => err == null, _ => ({
      statusCode: 500,
      body: "An unknown error occurred"
  }));

  on(err => err.message == 'Oh no!', err => ({
      statusCode: 400,
      body: err.message
  }));
}

@handle({ errorHandler })
public async handle() {
  throw new Error('Oh no!');
}

Injecting parameters

Extracting properties from the Lambda event and passing them to your function is streamlined like below:

Request Body

// Inject the request body, null if not present
@handle 
public handle(@requestBody body: any) {
  return body;
}

Parsing request body as JSON

By default, the request body will be parsed as JSON if the request Content-Type header is not present, or is set to application/json.

To control the behaviour of parsing the request body, an optional property parseJson can be passed to the @requestBody decorator:

@handle
public handle(@requestBody({ parseJson: true }) body: any){
    return body;
}

The parseJson property can be either boolean | string | (string | undefined)[], supporting the following behaviour:

• boolean: parse as JSON, if parseJson === true.
• string: parse as JSON, if the request Content-Type header is equal to the string.
• (string | undefined)[]: parse as JSON if the request Content-Type header matches any of the items in the array, following the same rules for a string as above. Additionally, the array can contain undefined items, indicating that the request body should be parsed as well when the request Content-Type header is not present.

For example:

@handle
public handle(@requestBody({ parseJson: [ 'application/json', 'application/detail+json', undefined ] }) body: any) {
    return body;
}

This will parse any request body for which the request Content-Type header is appication/json OR text/plain OR is omitted.

Supported content types

The @requestBody decorator can be augmented with a check on supported content types. The property for this is contentType. The contentType property can be either string | (string | undefined)[], following the same behaviour as the parseJson property.

For example:

@handle
public handle(@requestBody({ contentType: [ 'application/json', 'text/plain', undefined ] }) body: any) {
    return body;
}

This will check the request Content-Type header against the supported content types, and allow calls for which the request Content-Type header is application/json OR text/plain OR is omitted.

Otherwise, a 415 Unsupported Media Type response will be generated with the error message informing the client of the supported content types.

Parameter (path and query)

// Ex: GET /bikes/{id}/parts?filter=blue
@handle 
public handle(@pathParam('id') id: string, @queryParam('filter') filter: string) {
  return `ID: ${id}, filter: ${filter}`;
}

Authorizer context

// Inject the authorization context, null if not present
@handle 
public handle(@authorizer auth: any) {
  return auth;
}

Lambda event

// Inject the entire event
@handle 
public handle(@event event: any) {
  return event;
}

Lambda context

// Inject the lambda context
@handle 
public handle(@context context: Context) {
  return context;
}

Handler wrapper

For all this to work, the module file must export a handler property using the lambdaEntry() wrapper like below:

export class MyFunction {

  private param$?: Promise<string>;

  // The method marked as @init will be called before @handle.
  // Initialization is always synchronous, so promises should be dealt with
  // in the handler function
  @init
  public init() { 
    this.param$ = httpGet('my-param');
  }

  // The method marked as @handle will be called for each request and is always asynchronous.
  // This one here will return content type "application/json" because the return value is an object, along with status code 220
  @handle({ responseCode: 220 })
  public async handle() {
    const param = await this.param$;
    return {
      param
    };
  }
}

// Define the 'handler' like so
export const handler = lambdaEntry(MyFunction);

Content types

Only application/json and text/plain are properly supported out of the box.

• If a handler returns an object, it will be JSON stringified and the content type will be application/json. If a handler returns a string, the content type will be text/plain. XML is not supported, but can be implemented by returning a ResponseEntity instead and using your own serializer.
• Incoming request bodies are assumed to be JSON and will be parsed accordingly, unless the incoming content type is text/plain, in which case the request body will remain a string. If you want to send XML payloads, you may inject the @event into the handler and parse it yourself.

ResponseEntity

For complete control on what status code, body and headers are sent, return a ResponseEntity from the handler function:

export class MyFunction {
  
  @handle
  public async handle(): Promise<ResponseEntity<{  message: string }>> {
    return {
      statusCode: 201,
      body: { message: 'Created' },
      headers: {
        'X-Count': 10
      }
    };
  }
}

if omitted, the generic type of ResponseEntity will default to any, i.e. ResponseEntity is equal to ResponseEntity<any>.

Custom logging libraries

The lambdaEntry() wrapper itself will also do some logging, for example when errors are thrown by the handler method. This uses the console object by default, but can be overwritten:

import { logger } from 'my-logging-library';

// in this example, the order of the payload and message params is swapped for the real logger
class TslLoggerAdapter implements AwsLogger {
  info(message: string, payload?: any): void {
    logger.info(payload, message);
  }
  warn(message: string, payload?: any): void {
    logger.warn(payload, message);
  }
  error(message: string, payload?: any): void {
    logger.error(payload, message);
  }

  /**
  /* This method is called before the handler at each request, 
  /* and takes the request's event and context objects.
  /* Can be used to supply the logger with dynamic properties, such as a correlation ID.
  /* But it may as well be left empty
  **/
  setAwsRequest(event: any, context: Context): void {
    logger.withRequest(event, context); 
  }

}

export const tslLoggerAdapter = new TslLoggerAdapter();

Register it on the handle() decorator:

@handle({ logger: tslLoggerAdapter })

Creating a CDK construct

The @lambdaFunction() decorator writes some metadata to your class definition, including basic Lambda function settings and also a misc property that represents a map type for any custom properties. This metadata can be defined and retrieved as follows:

@lambdaFunction({
  name: 'MyFunction',
  entry: 'src/my-function.ts',
  timeoutMs: 5000,
  memoryMb: 128,
  misc: {
    myProperty: 20
  }
})
export class MyFunction {
  ...etc
}

StaticMetadata.get(MyFunction);
// {
//   lambdaFunctionProps {
//     name: 'MyFunction',
//     entry: 'src/my-function.ts',
//     memoryMb: 128,
//     timeoutMs: 5000,
//     misc: {
//        myProperty: 20
//     }
//   }
// }

Could not detect an AWS environment. Are you running a test?

The lambdaEntry() wrapper will look in the environment variables to check if it is running on AWS. If you are running tests, you can bypass this by setting the LAMBDA_TASK_ROOT environment variable to some value before the call to lambdaEntry() happens. This check is needed to prevent the function initialization from immediately running when importing the module, for example during a cdk synth.