Proxying Amazon AWS S3 Pre-Signed-URL Uploads Using Netlify Functions

<cfscript>

	// ... truncated code ...

	rc.apiResponse.data = getPreSignedUrls();

	// Intercept the response that contains our Amazon AWS S3 pre-signed URLs and
	// proxy them through a Netlify Lambda Function end-point.
	if ( uploadProxyService.isFeatureEnabled() ) {

		for ( var entry in rc.apiResponse.data ) {

			entry.signedurlOriginal = entry.signedurl;

			// Overwrite the pre-signed URL - the UploadProxyService is going to HEX-
			// encode the URL so that we don't have to worry about URL-encoding special
			// characters.
			entry.signedurl = ( "https://bennadel-s3-proxy.netlify.app/.netlify/functions/upload-to-s3?remoteUrl=" & uploadProxyService.encodeRemoteUrl( entry.signedurl ) );

		}

	}

	// ... truncated code ...

</cfscript>

// Using the dotenv package allows us to have local-versions of our ENV variables in a
// .env file while still using different build-time ENV variables in production.
require( "dotenv" ).config();

// Require core node modules.
var axios = require( "axios" ).default;
var Buffer = require( "buffer" ).Buffer;

// ----------------------------------------------------------------------------------- //
// ----------------------------------------------------------------------------------- //

// I am the Netlify Function handler.
// --
// NOTE: The function is ASYNC allowing us to AWAIT asynchronous methods within.
export async function handler( event, context ) {

	var corsHeaders = {
		"Access-Control-Allow-Origin" : process.env.NETLIFY_ACCESS_CONTROL_ALLOW_ORIGIN,
		"Access-Control-Allow-Headers": "Content-Type, Origin",
		"Access-Control-Allow-Methods": "OPTIONS, PUT"
	};

	// In the case of a CORS preflight check, just return the security headers early.
	if ( event.httpMethod === "OPTIONS" ) {

		return({
			statusCode: 200,
			headers: corsHeaders,
			body: JSON.stringify( corsHeaders )
		});

	}

	try {

		// Setup the PROXY configuration for our Axios request.
		var proxyHeaders = {
			"User-Agent": ( event.headers[ "user-agent" ] || "netlify-functions" ),
			"Content-Type": ( event.headers[ "content-type" ] || "application/octet-stream" )
		};
		// The remote-url is getting passed to this lambda function as a HEX-encoded
		// value so that we don't have to worry about URL-encoding special characters.
		var proxyUrl = Buffer
			.from( event.queryStringParameters.remoteUrl, "hex" )
			.toString( "utf8" )
		;
		// Netlify can pass the request body through as a String or a Buffer. As such, we
		// have to look at the instance-type to figure out how to generate a Buffer for
		// the subsequent Axios request.
		// --
		// NOTE: This behavior was recently changed in Netlify to make this possible.
		// Previously, this only worked for "image" content-types, but would break for
		// any payload that was of type "application/octet-stream".
		// --
		// Read More: https://community.netlify.com/t/changed-behavior-in-function-body-encoding/19000
		var proxyBody = ( event.body instanceof Buffer )
			? event.body
			: Buffer.from( event.body, ( event.isBase64Encoded ? "base64" : "utf8" ) )
		;

		var proxyResponse = await axios({
			method: "put",
			url: proxyUrl,
			headers: proxyHeaders,
			data: proxyBody
		});

		console.log({
			successData: proxyResponse.data,
			successStatus: proxyResponse.status,
			successStatusText: proxyResponse.statusText,
			successHeaders: proxyResponse.headers
		});

		return({
			statusCode: proxyResponse.status,
			headers: {
				...proxyResponse.headers,
				...corsHeaders
			},
			body: proxyResponse.data
		});

	} catch ( error ) {

		console.error({
			errorData: error.response.data,
			errorStatus: error.response.status,
			errorHeaders: error.response.headers
		});

		return({
			statusCode: 500,
			headers: {
				"Content-Type": "text/plain",
				...corsHeaders
			},
			body: JSON.stringify( "Server Error" )
		});

	}

}