Office JS internetHeaders.getAsync incorrect AsyncResult type

[alexm-plutoflume]

Type of issue

Other (describe below)

Feedback

The type signature for the getAsync method for Office JS internet headers indicates that the async result should be Record<string, string>>.

We found in Classic Outlook for Windows this method can return null if the header requested doesn't exist. The type signature does not indicate that this method can return null. The typescript typestubs provided by MSFT also do not indicate this value can be null.

For Mac, New Outlook for Windows, and Outlook on the Web, this same scenario returns an empty Record which is what we would expect.

Is this a bug with Classic Outlook for Windows?

Page URL

https://learn.microsoft.com/en-us/javascript/api/outlook/office.internetheaders?view=outlook-js-preview

Content source URL

https://github.com/OfficeDev/office-js-docs-reference/blob/live/docs/docs-ref-autogen/outlook/outlook/office.internetheaders.yml

Author

@o365devx

Document Id

bd5143a2-6f72-130c-ffed-d134ec3ec2b9

Platform Id

b95b41a3-e408-b3dc-fcab-7eda122d4924

[samantharamon]

Hi @alexm-plutoflume,

Thanks for raising this. When you get a chance, could you please share the code snippet you're testing? When I tested this scenario in classic Outlook on Windows, new Outlook on Windows, and Outlook on the web, the getAsync returns an empty Record.

[alexm-plutoflume]

Hiya @samantharamon , here's a code snippet that I can reproduce with. For context - this is a smart alerts OnMessageSend handler. My classic outlook version is Microsoft 365 MSO (Version 2511 Build 16.0.19426.20186) 64-bit

In this code snippet we wrap the office js api in a promise so we can use async/await which is pretty standard.

export const getInternetHeadersAsync = (headers: string[]): Promise<Record<string, string>> => {
  return new Promise<Record<string, string>>((resolve, reject) => {
    Office.context.mailbox.item?.internetHeaders.getAsync(
      headers,
      (asyncResult: Office.AsyncResult<Record<string, string>>) => {
        if (asyncResult.status === Office.AsyncResultStatus.Failed) {
          reject(asyncResult.error);
        } else {
          resolve(asyncResult.value);
        }
      }
    );
  });
};

const onMessageSendHandlerImpl = async (event: Office.MailboxEvent) => {
  try {
    const headers = await getInternetHeadersAsync(["x-test-header"]);
    console.log(headers);
    // This will throw an error
    const header = headers["x-test-header"];
  } catch (e) {
    console.log(e);
  } finally {
    event.completed();
  }
};

And these are the corresponding runtime logs

18/12/2025 17:33:37.252	Verbose	Runtime	[Console] [Log] RunLaunchEventHandlerFunctionWithData was invoked with funcName = onMessageSendHandler		
18/12/2025 17:33:37.257	Verbose	Runtime	[Console] [Log] null		
18/12/2025 17:33:37.257	Verbose	Runtime	[Console] [Log] [TypeError: Cannot read properties of null (reading 'x-test-header')]		
18/12/2025 17:33:37.257	Verbose	Runtime	[Console] [Log] LaunchEventObj.completed was invoked: 5		
18/12/2025 17:33:37.259	Verbose	Runtime	[Console] [Log] Event completed message sent to host!