Cancel save based on the result of async operation (Example 1)

AccountForm.js

if (typeof (MEA) == "undefined") { MEA = {} };
if (typeof (MEA.Account) == "undefined") { MEA.Account = {} };
if (typeof (MEA.Account.AccountForm) == "undefined") { MEA.Account.AccountForm = {} };

MEA.Account.AccountForm = {
    onLoad: function (executionContext) {
        var formContext = executionContext.getFormContext();
        formContext.data.entity.addOnSave(
            (() => {
                var asyncSaveValidation = (saveCtx) => {
                    return new Promise((resolve, reject) => {
                        //timeout 10 sec
                        //reject("error");
                    });
                };
                return async (saveCtx) => {
                    saveCtx.getEventArgs().preventDefaultOnError();
                    let result = await asyncSaveValidation(saveCtx);
                }
            }
            )());
    }
}