CORS for Serverless

rootHandler.ts

import { APIGatewayProxyHandler, Context, Callback } from 'aws-lambda';

interface LambdaResponse {
    body: any,
    statusCode?: number
}

type Lambda = (event?: any, context?: Context, callback?: Callback) => Promise<LambdaResponse>

export interface LambdaConfig {
    handler: Lambda
    allowCredentials: boolean
}

const ALLOWED_ORIGINS = [ ]
const generateCORSHeaders = (event:any, config: LambdaConfig) => {
    const requestOrigin = (event && event.headers && event.headers.origin || "")
    const allowedOrigin = ALLOWED_ORIGINS.find( origin => origin === requestOrigin)
    if (allowedOrigin) {
        let headers = {
            "Access-Control-Allow-Origin": allowedOrigin
        }
        if (config.allowCredentials)
            headers["Access-Control-Allow-Credentials"] = true
        return headers
    }
    else return { }
}

export default (config: LambdaConfig):APIGatewayProxyHandler => async (event, context, cb) => {
    console.log("Request Received");
    const response = await config.handler(event, context, cb);
    console.log("Response:", response);
    return {
        body: JSON.stringify(response.body),
        statusCode: response.statusCode || 200,
        headers: generateCORSHeaders(event, config)
    }
}

//Used like so, where handler is your custom lambda function
const entry = rootHandler({
  handler: async () => ({ body: "Hello World" }),
  withCredentials: true
})