In Excel Online, OfficeJS API is not passing the host_Info_ parameter anymore to Excel Add-In - excel

Question

I don't know when this started happening or if things have changed but the _host_Info param is not being passed by the framework to my Excel Add-In in Excel Online.
It passes an empty "et=" parameter which is OK in this case since I'm in dev mode. The Excel desktop client is still passing it.
This is required by my add-ins to switch functionality between Excel Online and Excel for Windows.
I've checked the docs and can't find that anything has changed.

Answer 1

To append to Sudhi's answer: with the official API coming as per the link in Sudhi's answer, we've gone ahead and also added a "shim" for the API in the OfficeJsHelpers library.
The shim still uses the workaround code that Sudhi mentions -- window.sessionStorage['hostInfoValue'] -- but it wraps it in an API very similar to what is coming in the official Office.js. Once the official API is available, we'll switch the shim code to make use of that instead. The beauty of this approach is that if you use OfficeJsHelpers via an NPM package, all you have to do is update your package dependency, and you'll suddenly go from an unofficial and potentially-fragile workaround to an API that relies on 100% officially-exposed properties -- all without changing your own code! Likewise, if at that time you decide to switch back to using the official Office.js version, the similarity of the APIs (essentially just namespace differences) should make it trivial to switch over the implementations.
The helper APIs are OfficeHelpers.Utilities.host (which will return WORD, EXCEL, etc.) and OfficeHelpers.Utilities.platform (which will return IOS, PC, OFFICE_ONLINE, or MAC). The constants are defined in OfficeHelpers.HostType and OfficeHelpers.PlatformType.
You can find the NPM package at https://www.npmjs.com/package/#microsoft/office-js-helpers, and either install it via NPM, or use a CDN like Unpkg to quickly try it out: https://unpkg.com/#microsoft/office-js-helpers#0.4.2/dist/office.helpers.min.js
console.log(OfficeHelpers.Utilities.host);
console.log(OfficeHelpers.Utilities.platform);
if (OfficeHelpers.Utilities.platform === OfficeHelpers.PlatformType.OFFICE_ONLINE) {
console.log("Yep, I'm on the web client");
}
I hope you find the above helpful as an interim measure, and we'll be sure to update this StackOverflow thread (and the OfficeJsHelpers code) once the official API is released.

Answer 2

Jim and others who may be affected by this change: it is unfortunate that an unrelated change meant to improve the add-in experience caused the query string parameter in online platform to be removed. Note that URL query parameters and session storage settings are used to launch and setup the add-in environment and are not meant for developer consumption. However, we understand the importance of this to the developers as described in this thread.
Hence, we are adding formal APIs to make this information available. Please see the specs that describes upcoming APIs: https://github.com/OfficeDev/office-js-docs/tree/ContextAdditions_OpenSpec
Until the APIs become available (should be very soon), you could use the following temporary workaround. The same query host_info_ parameter value is available in the following variable: window.sessionStorage.hostInfoValue. Please note that this should be considered a temporary measure and you should switch to using the formal API as soon as they are made available. I'll update the this thread when the API is released, which should be available for all supported Office versions.
If you have any comments, please leave your comments on the specs directly in Github specification branch using the links provided.

Answer 3

UPDATE:
Please see my answer above, instead (https://stackoverflow.com/a/40963500/678505), as the scenario is now possible.
[Old response:]
Jim, could you describe your scenario more? Why is it that you need to distinguish between Online and Desktop?
For the disappearance of host_info_: you won't find it in the docs, because host_info_ was never in the docs. Anything not documented is an internal API, which might change as needed by the internal workings of Office.js. We take back-compat very seriously, but only for actual official documented APIs.
Without an API, there's not much you can do to tell the two apart (or rather, not without relying on other bits of internal workings which might also change over time). We've discussed this internally before, but found it surprisingly difficult to point to a particular scenario where the platform information was necessary and legitimate ("legitimate" in the sense that if you're using platform information to check whether an API is available, for example, that's precisely what we don't want you to do; we want you to use Office.context.requirements.isSetSupported instead).
If you can share your scenario details, I might be able to advise an alternate way of determining whatever you need, or have a solid scenario to bring back to the team.