How to get formContext in Ribbon command of Dynamics 365 9.0

After passing the primaryControl as @scott-durow suggested, It is best to not use primaryControl.getFormContext() and instead use the primaryControl as though it is the formContext.

According to the documentation (1/2/2019): https://docs.microsoft.com/en-us/dynamics365/customer-engagement/developer/customize-dev/pass-dynamics-365-data-page-parameter-ribbon-actions#form-and-grid-context-in-ribbon-actions, one should perform operations on the primaryControl as though it is the formContext.

function mySampleFunction(primaryControl) {
    var formContext = primaryControl;
    // Perform operations using the formContext object
}

But, the key piece of the example provided is this: // Perform operations using the formContext object being the key (no idea why they added the var formContext = primaryControl line, imo, it would have been clearer if they had instead just shown an example: primaryControl.getAttribute('xxxx');

I suspect primaryControl.getFormContext() code started getting used because thats how you get the formContext when working with forms (https://docs.microsoft.com/en-us/dynamics365/customer-engagement/developer/clientapi/clientapi-form-context#using-the-formcontext-object-instead-of-the-xrmpage-object).

The problem with using primaryControl.getFormContext() is that it works with the normal Web interface, but breaks with the UCI. But if you use primaryControl as though it is the form-context, then it works for both the legacy web-client and uci interfaces.

Here is a function that I use:

function getFormContext(executionContext) {
     var formContext = null;
     if (executionContext !== null) {
         if (typeof executionContext.getAttribute === 'function') {
             formContext = executionContext; //most likely called from the ribbon.
         } else if (typeof executionContext.getFormContext === 'function' 
                 && typeof(executionContext.getFormContext()).getAttribute === 'function') {
            formContext = executionContext.getFormContext(); // most likely called from the form via a handler
         } else {
            throw 'formContext was not found'; //you could do formContext = Xrm.Page; if you like.
        }
    }
    return formContext;
}

I was having the same issue as well. What I found out was, there was an error in Microsoft doco. Please follow whatever Scott mentioned passing CRM Parameter from ribbon command action. In javascript function, please try below to get form context

var formContext = primaryControl.getFormContext();

this fixed my issue.

As per https://docs.microsoft.com/en-us/dynamics365/get-started/whats-new/customer-engagement/important-changes-coming#some-client-apis-are-deprecated you pass it as the PrimaryControl parameter.

So if you pass the PrimaryControl as the second parameter to a command function like this you can use

arguments[1].getAttribute(…)