amplify-alpha: custom_response_headers cannot be empty

[bobveringa-sf]

Describe the bug

In CDK 2.202.0, PR #31771 was introduced, which added support for appRoot in custom response headers for Amplify. This PR caused issues in our application for some uses of the amplify.App construct. Specifically, the issue appeared whenever custom_response_headers was an empty list.

Regression Issue

• Select this option if this issue appears to be a regression.

Last Known Working CDK Library Version

2.201.0

Expected Behavior

Run cdk synth without errors

Current Behavior

Running cdk synth fails with the following error.

jsii.errors.JavaScriptError:
  TypeError: Cannot read properties of undefined (reading 'appRoot')
      at renderCustomResponseHeaders (REDACTED\node_modules\@aws-cdk\aws-amplify-alpha\lib\app.js:269:41)
      at new App (REDACTED\node_modules\@aws-cdk\aws-amplify-alpha\lib\app.js:103:58)
      at new App (REDACTED\node_modules\aws-cdk-lib\core\lib\prop-injectable.js:1:488)
      at Kernel._Kernel_create (REDACTED\Local\Temp\tmpzcf1ob0l\lib\program.js:548:25)
      at Kernel.create (REDACTED\Local\Temp\tmpzcf1ob0l\lib\program.js:218:93)
      at KernelHost.processRequest (REDACTED\Local\Temp\tmpzcf1ob0l\lib\program.js:15479:36)
      at KernelHost.run (REDACTED\Local\Temp\tmpzcf1ob0l\lib\program.js:15439:22)
      at Immediate._onImmediate (REDACTED\Local\Temp\tmpzcf1ob0l\lib\program.js:15440:45)
      at process.processImmediate (node:internal/timers:485:21)

Reproduction Steps

My example is based on Python, but given how simple it is, it should not be a problem to reproduce this in other languages.

app = amplify.App(
    self, 'App',
    custom_response_headers=[],
)

Possible Solution

#31771 Added (among other lines) the line
const hasAppRoot = customHeaders[0].appRoot !== undefined; link.

My guess is that taking index 0 of an empty list is what is causing the error. In this case a possible solution would be to add a simple check that the list has more than 0 entries.

Additional Information/Context

No response

AWS CDK Library version (aws-cdk-lib)

2.202.0

AWS CDK CLI version

2.1029.4 (build 09c0061)

Node.js Version

v22.20.0

OS

Windows 11

Language

Python

Language Version

No response

Other information

No response

[pahud]

investigating

[pahud]

Hi @bobveringa-sf,

Thank you for reporting this regression! Based on our analysis, it appears that PR #31771 introduced a bug in the renderCustomResponseHeaders function that doesn't handle empty arrays properly. I am still working on the reproduction through real CDK deployment.

The issue occurs at line 598 where the code attempts to access customHeaders[0].appRoot without first checking if the array is empty:

const hasAppRoot = customHeaders[0].appRoot !== undefined;

When customResponseHeaders is an empty array, customHeaders[0] is undefined, which causes the TypeError you're seeing.

Suggested Fix:

I believe the function should check if the array is empty before accessing the first element. Something like:

function renderCustomResponseHeaders(customHeaders: CustomResponseHeader[], scope: IConstruct): string {
  if (customHeaders.length === 0) {
    return '';
  }
  const hasAppRoot = customHeaders[0].appRoot !== undefined;
  // ... rest of the function
}

This would be a straightforward fix that should resolve the issue. I will cut an immediate PR to address this and callout the team for the review. I'll post the comments here if there's any update.

Thank you.

[pahud]

Reproduction results:

• v2.201.0: ✅ Works
• v2.202.0: ❌ Broken
• v2.219.0: ❌ Still broken (17 releases later)

PR WIP.