Writing Backend Code

Overview

Backend function code executes in a serverless environment associated with your 8th Wall account. All backend functions must export a top level async method called handler, which is the entry point into the backend function.

Example entry file code:

const handler = async (event: any) => {
  // Custom backend code goes here

  return {
    body: JSON.stringify({
      myResponse,
    }),
  }
}

export {
  handler,
}

Client Method

When you create a backend function, a client method is automatically created for you. This client method is a wrapper around fetch, meaning you can pass the same arguments to this function as you would with a normal fetch call. See Fetch API for more details.

This client method is how you send requests from module client code to the backend function.

Function Event Parameters

The handler method is invoked with an event object each time the client method is called. event has the following properties:

Property	Type	Description
path	string	The URL path passed to the client method ('/getUser/foo' , '/checkAnswer', etc).
body	string	Call JSON.parse(event.body) to get transform the body into an object.
httpMethod	string	The HTTP method used to call the backend function. One of 'GET', 'PUT', 'POST', 'PATCH', 'DELETE'.
queryStringParameters	Record<string, string>	Key/value pairs containg the query string parameters from the request.
headers	Record<string, string>	Key/value pairs containing request headers.

Return Object

All properties are optional.

Property	Type	Description
statusCode	number	The status code of the response. Default is 200.
headers	Record<string, string>	The headers associated with the response.
body	string	The JSON.stringify()'d body object associated with the response.

Error Handling

If the backend function throws an uncaught exception, the function will return statusCode: 500 with an error object in the response body.

If you own the module and are in development mode, the error object will contain name, message and stack:

{error: {name: string, message: string, stack: string}}

Example:

{
  "error": {
    "name": "TypeError",
    "message": "Cannot read properties of undefined (reading 'foo')",
    "stack": "TypeError: Cannot read properties of undefined (reading 'foo')\n at call (webpack:///src/index.ts:8:24)\n ...
  }
}

For non-development mode, the error object will not contain a name or stack property and the message will be a generic "Internal Server Error".

Pinning Targets

Please refer to https://www.8thwall.com/docs/guides/modules/pinning-targets/ for full details on module pinning targets.

When pinning to a Version, Allowed Updates must be set to None

When pinning to a Commit, select a specific commit. Latest is not supported.