@lambda-middleware/compose/README.md

A compose function for functional lambda middleware.

# @lambda-middleware/compose

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A compose function for lambda middleware. This is a pure convenience copy of [ramda's compose](https://ramdajs.com/docs/#compose), using it directly or using other compose functions works as well.

## Lambda middleware

This middleware is part of the [lambda middleware series](https://dbartholomae.github.io/lambda-middleware/). It can be used independently.

## Usage

```typescript
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](https://github.com/microsoft/TypeScript/issues/29904) 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`:

```ts
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
);
```