Service worker: how the site can work in the absence of a network

Useful to know

Some web applications can display some of the content even when the connection to the Internet breaks. One of the tools for this is service worker: JavaScript code, which works in a separate context and can be involved in processing resource requests. It is associated with a certain origin and path. [1] It is important to distinguish the possibility from the finished result. The presence of such a file in the project does not in itself prove what exactly the visitor will see without a network.

Service worker does not have direct access to DOM on the page and is designed for asynchronous operation. It can intercept queries and return appropriate responses according to the written logic. Its use usually requires a protected context, with local development having exceptions. [1] This explains why the technology is not just another script for changing the text on the page. Its role is around access to resources and the application life cycle.

The Cache interface offers storage of query and response pairs. Different named caches can be supported, and the responsibility for updating and removing content lies with the application. Records are not automatically replaced just because the server already has another version. [2] Therefore, the local cache must have a development plan. Otherwise, a good idea of networkless access can lead to the display of old files that no longer match the rest of the site.

The browser places restrictions on the space used and can remove locally saved data. Cache is not a promise of an unconditional constant presence of any resource. [2] A good offline view therefore needs to explain what is available and what a connection requires. If an article has been saved, it can be displayed; if its image is missing, the interface must have a reasonable response. The accidental presence of several files is not enough for a consistent user experience.

Let's imagine a travel guide. The author can choose in advance to preserve the main texts, and the remaining resources to search when needed. Another option is to keep only what is already open. These two approaches give different expectations for the person who leaves without the Internet. This is a design example: the right strategy depends on the volume, the frequency of changes and the promise of the product. Each "accessible offline" statement must specify the actual scope of this accessibility.

Checking includes first visit, re-open, disconnect and move to a new version. It is also important to try a case where the expected local resource is missing. This shows whether the application recognizes different states, instead of showing a single common blank screen. Service worker is a useful basis for a more sustainable web interface when the developer consciously manages the source, freshness, and availability of any response that reaches the visitor.