Multiple Mailchimp JSONP function calls lead to "Unexpected identifier" error

[karlhorky]

Thanks for the library!

With the following JSONP response (script body) from Mailchimp:

jsonp_1720618209454_25752({"result":"error","msg":"Too many subscribe attempts for this email address. Please try again in about 5 minutes. (#6592)"})jsonp_1720618209454_25752({"result":"error","msg":"abc@gmail.co is an invalid email address and cannot be imported."})

fetch-jsonp times out, with the message (details sanitized):

Error: JSONP request to https://xxx.us20.list-
manage.com/subscribe/post-json?u=xxx&id=xxx&EMAIL=abc%40gmail.co timed out

Looking in the DevTools console, it appears that the problem happened because of the jsonp_1720618209454_25752 following directly after the closing parenthesis:

Uncaught SyntaxError: Unexpected identifier 'jsonp_1720618209454_25752'

[karlhorky]

I think this may be actually 2 bugs though:

1. fetch-jsonp calls resolve() and removes the script on the first usage of the function https://github.com/camsong/fetch-jsonp/blob/85d90c7e0dd7fb12f40f6eb01bb1553ed2960f58/src/fetch-jsonp.js#L40-L52
2. A Mailchimp bug, that they are not including a semicolon between the function calls (or maybe , or || or && instead, if it's supposed to be a JavaScript expression)

[karlhorky]

Looking into JSONP more, it seems like it's pretty standard to have only a single function call.

@camsong So if that's the convention, I'm happy to have this issue closed - because it would be only that Mailchimp is breaking that convention.

[karlhorky]

To work around this issue, we ended up writing our own Next.js Route Handler which passes along the request query parameters and fixes the Mailchimp response (only shows the first error):

app/api/mailchimp-subscriptions/route.ts

import { NextRequest, NextResponse } from 'next/server';

type MailchimpSubscriptionsResponseBodyGet = string;

// Fix broken Mailchimp JSONP responses (double function calls)
// https://github.com/camsong/fetch-jsonp/issues/72#issuecomment-2220956128
export async function GET(
  request: NextRequest,
): Promise<NextResponse<MailchimpSubscriptionsResponseBodyGet>> {
  const url = new URL(request.url);

  url.hostname = 'xxx.us20.list-manage.com';
  url.pathname = '/subscribe/post-json';
  url.port = '';

  const searchParams = request.nextUrl.searchParams;
  const callbackFunctionName = searchParams.get('c');

  try {
    const response = await fetch(url.toString());

    if (!response.ok) {
      return new NextResponse(
        `${callbackFunctionName}({"result":"error","msg":"Invalid response status"})`,
      );
    }

    const textResponse = await response.text();

    const firstJsonpFunctionCall = textResponse.match(
      new RegExp(`^${callbackFunctionName}\\(\\{[^}]+\\}\\)`),
    );

    if (!firstJsonpFunctionCall) {
      return new NextResponse(
        `${callbackFunctionName}({"result":"error","msg":"Invalid response format"})`,
      );
    }

    return new NextResponse(firstJsonpFunctionCall[0], {
      headers: {
        'Content-Type': 'application/javascript',
      },
    });
  } catch (error) {
    return new NextResponse(
      `${callbackFunctionName}({"result":"error","msg":"${(error as Error).message}"})`,
    );
  }
}

Then just change your call to fetchJsonp() to use the new Route Handler instead of Mailchimp directly:

const response = await fetchJsonp(
-  `https://xxx.us20.list-manage.com/subscribe/post-json?u=xxx&id=xxx&${new URLSearchParams({ EMAIL: email }).toString()}`,
+  `/api/mailchimp-subscriptions?u=xxx&id=xxx&${new URLSearchParams({ EMAIL: email }).toString()}`,
  {
    jsonpCallback: 'c',
  },
);