1. A method, comprising: receiving a user action script, a first callback queue, and an asynchronous operation queue generated based on a first execution instance of a web application in a first computing environment; simulating, by operation of a computer processor, the first execution instance of the web application, comprising: invoking a second execution instance of the web application in a second computing environment, different than the first computing environment; simulating, during the second execution instance of the web application, each user action of a plurality of user actions specified in the user action script, based on a plurality of callback functions specified in the first callback queue; simulating, during the second execution instance of the web application, each asynchronous operation of a plurality of asynchronous operations specified in the asynchronous operation queue based on the plurality of callback functions specified in the first callback queue; and generating, during the second execution instance of the web application, a second callback queue based on the simulated plurality of user actions and asynchronous operations; and upon determining that an order of the plurality of callback functions specified in the first callback queue matches an order of a plurality of callback functions specified in the second callback queue, returning an indication that the simulation of the first execution instance of the web application was successful.