middleware-composer

Compose middleware function originated from thread on github https://github.com/dbartholomae/lambda-middleware/pull/73

This package does not provided it's own ready to use middlewares, you need to write all by your own.

However you may want to consider using those from lambda-middleware if you have reasons to not write your own middleware functions. (I don't recommend that since those packages are not so commonly used)

Warning

event may have unknown type and it's known typescript bug which often happens whhile composing. To fix types remove event variable and write it back. (ctrl + x and ctrl + v)

Examples

Example of usage composeMiddlewares function

export const handle = composeMiddlewares(
  middlewareErrorHandler(),
  middlewareLogger(),
  middlewareJsonParser(),
  middlewareValidator(lambdaSchemaPackageActivate), // infer types from validator
  middlewareLoadDependencies(["repositoryPackage"]), // infer types from DI
  async (event) => {
    const existingPackage = await event.dependencies.repositoryPackage.findPackage({
      packageName: event.jsonBody.packageName,
    });
    if (existingPackage === null) {
      throw new ConflictError("package must exists in database");
    }
    if (existingPackage.packageStatus === PackageStatus.active) {
      throw new ConflictError("package.packageStatus must not equal active");
    }
    await event.dependencies.repositoryPackage.changePackageStatus({
      packageName: event.jsonBody.packageName,
      packageStatus: PackageStatus.active,
    });
    return awsLambdaResponse(StatusCodes.OK, {}, {});
  },
);

Example of middleware for logging events from within lambda using winston

import type { APIGatewayProxyEvent, APIGatewayProxyResult, Context } from "aws-lambda";
import { winstonLogger } from "../logger/logger";

export const middlewareLogger =
  <TApiGatewayProxyEvent extends APIGatewayProxyEvent>() =>
  (
    handler: (event: TApiGatewayProxyEvent, context: Context) => Promise<APIGatewayProxyResult>,
  ): ((event: TApiGatewayProxyEvent, context: Context) => Promise<APIGatewayProxyResult>) =>
  async (event, context) => {
    winstonLogger.info(`Incoming lambda event -> ${JSON.stringify(event)}`);
    const result = await handler(event, context);
    winstonLogger.info(`Outgoing lambda result -> ${JSON.stringify(result)}`);
    return result;
  };

Example of middleware implementation using zod as validation library to pass types from the validator into handler automatically.

import type { APIGatewayProxyEvent, APIGatewayProxyResult, Context } from "aws-lambda";
import type { ZodError, ZodTypeAny } from "zod";
import { BadRequestError } from "../error/bad-request.error";
import type { zod } from "../zod";

export type ValidatedFields = "headers" | "jsonBody" | "queryStringParameters" | "pathParameters";

export type EndpointSchema = {
  [key in ValidatedFields]: ZodTypeAny;
};

export const middlewareValidator =
  <TApiGatewayProxyEvent extends APIGatewayProxyEvent, TEndpointSchema extends EndpointSchema>(
    endpointSchema: TEndpointSchema,
  ) =>
  (
    handler: (
      event: TApiGatewayProxyEvent & {
        jsonBody: zod.infer<TEndpointSchema["jsonBody"]>;
        headers: zod.infer<TEndpointSchema["headers"]>;
        pathParameters: zod.infer<TEndpointSchema["pathParameters"]>;
        queryStringParameters: zod.infer<TEndpointSchema["queryStringParameters"]>;
      },
      context: Context,
    ) => Promise<APIGatewayProxyResult>,
  ): ((
    event: TApiGatewayProxyEvent & {
      jsonBody: zod.infer<TEndpointSchema["jsonBody"]>;
      headers: zod.infer<TEndpointSchema["headers"]>;
      pathParameters: zod.infer<TEndpointSchema["pathParameters"]>;
      queryStringParameters: zod.infer<TEndpointSchema["queryStringParameters"]>;
    },
    context: Context,
  ) => Promise<APIGatewayProxyResult>) =>
  async (event, context) => {
    const errors = Object.entries(endpointSchema).reduce((acc, [key, schema]) => {
      const validationResult = schema.safeParse(event[key as ValidatedFields]);
      if (validationResult.success === false) {
        acc[key as ValidatedFields] = validationResult.error.issues;
        return acc;
      } else {
        acc[key as ValidatedFields] = [] as any;
      }
      event[key as ValidatedFields] = validationResult.data as any;
      return acc;
    }, {} as { [key in ValidatedFields]: ZodError["issues"] });
    if (Object.values(errors).flat().length > 0) {
      throw new BadRequestError(errors);
    }
    const result = await handler(event, context);
    return result;
  };