Minimum example for working with SP webhooks from Azure Functions

azure-function-webhook.js

module.exports = function (azureContext, req) {

    if (req.query.validationtoken) {
        azureContext.log('Validation token received: ' + req.query.validationtoken);
        azureContext.res = {
            body: req.query.validationtoken,
            isRaw: true
        };
        azureContext.done();
        return;
    }

    azureContext.log("Request body:", req.body);
	azureContext.done();
});

You can also use sphooks if you prefer cli tools.