lard-lambda-handler

A collection of wrapper functions for building AWS Lambda handlers from promise-generating functions.

lambda(fn)

Returns an event handler function for use with AWS Lambda. The fn function provided is the handler implementation written in the promise-based style. fn takes the event and context as its first two arguments, and is expected to return a promise whose resolved value will be passed as the successful result of the lambda callback. Any error that is thrown within the promise or chain of promises will be caught and returned as the error result of the Lambda callback. If function throws an error outside of the promise chain, it will also be caught and handled.

Example:

const lambda = require('lard-lambda-handler').lambda;
module.exports.handler = lambda(event => SomeService.fetchSomething(event.someId));

api(fn)

Similar to lambdaHandler but converts the results of the promise into an API Gateway response, which looks something like:

{
  "statusCode": 200,
  "headers": {
    "Content-Type": "text/html"
  },
  "body": "{ \"foo\": \"bar\" }"
}

The response is constructed from the promise using the following rules:

• If the promise is fulfilled with the value result...
  • If result.statusCode is set, use result as the whole response.
  • Otherwise, create a response with result as the body, a default statusCode of 200, and empty headers.
• If the promise is rejected with error...
  • If error is an instance of HttpError, use its statusCode with the corresponding error message as body.
  • Otherwise, return a generic server error response with a statusCode of 500.

In any case, if body is not a string, it will be JSON-stringified. This allows you to return objects from your handler and have them automatically converted for API Gateway.

Example:

const httpErrors = require('http-errors');
const api = require('lard-lambda-handler').api;

module.exports.handler = api(event => {
	const docId = event.pathParameters.documentId;
	return SomeService.fetchDocument(docId)
		.then(document => {
			if (!document) {
				throw new httpErrors.NotFound(`No document found with id ${docId}`);
			}
			if (document.secure) {
				return {
					statusCode: 403,
					body: `Document with id ${docId} is secure`,
				};
			}
			return document;  // default to statusCode 200
		});
});

postFormUrlEncoded(fn)

Preprocesses an event.body64 base64-encoded form-url-encoded input into an event.body object before running fn. Returns the raw response from fn.

postRaw(fn)

Preprocesses an event.body64 base64-encoded raw post input into an event.body object before running fn. Returns the raw response from fn.