Isomorphic JavaScript works in the context of a single-page application (SPA). In a typical SPA, most of the application logic, including routing, is encapsulated in a bundled JavaScript file that is sent to the client. While this frees up the server, as it does not have to handle so many requests, it also makes the initial load slow for the client, as the entire application needs to be sent over to the client.

With Isomorphic JavaScript, when a client web page is first requested from the server, the view of the page is generated on the server, similar to server-side dynamic web pages, and sent over to the client. The client can then render the view immediately. After the initial view is rendered, the complete SPA is downloaded in the background, and subsequent actions are handled client-side.