@lambda-middleware/compose

A compose function for lambda middleware. This is a pure convenience copy of ramda’s compose, using it directly or using other compose functions works as well.

Lambda middleware

This middleware is part of the lambda middleware series. It can be used independently.

Usage

import { compose } from "@lambda-middleware/compose";
import { PromiseHandler } from "@lambda-middleware/utils";
import { Context, ProxyHandler, APIGatewayEvent } from "aws-lambda";

// This is your AWS handler
async function helloWorld(): Promise<object> {
  return {
    message: "Hello world!",
  };
}

// Write your own middleware
const stringifyToBody = () => (
  handle: PromiseHandler<APIGatewayEvent, object>
) => async (event: APIGatewayEvent, context: Context) => {
  const response = await handle(event, context);
  return {
    body: JSON.stringify(response),
  };
};

const addStatusCode = (statusCode: number) => (
  handle: PromiseHandler<APIGatewayEvent, { body: string }>
) => async (event: APIGatewayEvent, context: Context) => {
  const response = await handle(event, context);
  return {
    ...response,
    statusCode,
  };
};

// Wrap the handler with the middleware.
// With compose you can wrap multiple middlewares around one handler.
export const handler: ProxyHandler = compose(
  addStatusCode(200),
  stringifyToBody()
)(helloWorld);

Usage in TypeScript strict mode

There’s a known issue with TypeScript that pipe and compose functions cannot infer types correctly if the innermost function is generic (in this case the last argument to compose). To get around it, this package also exports composeHandler:

import "reflect-metadata";

import { classValidator } from "@lambda-middleware/class-validator";
import { compose } from "@lambda-middleware/compose";
import { errorHandler } from "@lambda-middleware/http-error-handler";
import { IsString } from "class-validator";
import { APIGatewayProxyResult } from "aws-lambda";

class NameBody {
  constructor(firstName: string, lastName: string) {
    this.firstName = firstName;
    this.lastName = lastName;
  }

  @IsString()
  public firstName: string;

  @IsString()
  public lastName: string;
}

async function helloWorld(event: {
  body: NameBody;
}): Promise<APIGatewayProxyResult> {
  return {
    body: `Hello ${event.body.firstName} ${event.body.lastName}`,
    headers: {
      "content-type": "text",
    },
    statusCode: 200,
  };
}

export const handler = composeHandler(
  errorHandler(),
  classValidator({
    bodyType: NameBody,
  }),
  // The following function solves the type trouble:
  helloWorld
);