An Http Triggered Javascript Azure Function to process event grid events. If it's a validation event, send back the validationCode to event grid.

EventGridHandlerHttpTrigger.js

module.exports = function (context, req) {

    for (var events in req.body) {
        var body = req.body[events];
        
        if (body.data && body.eventType == "Microsoft.EventGrid.SubscriptionValidationEvent") {
            context.log("event grid validation event, validation code: " + body.data.validationCode);

            //validate header
            if(req.headers['aeg-event-type'] && req.headers['aeg-event-type'] == 'SubscriptionValidation')
            {            
                // Do any additional validation (as required) and then return back the below response
                var code = body.data.validationCode;
                context.res = { status: 200, body: { "ValidationResponse": code } };
            }
            else{
                context.res = { status: 400, body: "Validation header is missing." };
            }
        }
        else{
            //for all other events, do the processing you need to do, such as call another azure function
            // or a logic app, send alerts, or emails etc.
            
            // create a response body, with the properties you're interested in, or just send the event in response.
            let responseBody = {                 
                    "eventType": body.eventType,
                    "operationName": body.data.operationName,
                    "resourceProvider": body.data.resourceProvider,
                    "resourceUri": body.data.resourceUri,
                    "status": body.data.status,  
                    "subject": body.subject
                    "subscriptionId": body.data.subscriptionId,  
                    "tenantId": body.data.tenantId,
                };
                
            context.log("Resoonse body", responseBody);
            context.res = { status: 200, body: responseBody };
                
        }
    }
    context.done();
};