Professional JavaScript for Web Developers, 4th Edition

27Workers

WHAT'S IN THIS CHAPTER?

• Introduction to workers
• Running background tasks with dedicated workers
• Using with shared workers
• Managing requests with service workers

The statement “JavaScript is single-threaded” is practically a mantra for the frontend development community. This assertion, although it makes some simplifying assumptions, effectively describes how the JavaScript environment generally behaves inside a browser. Therefore, it is useful as a pedagogical tool for helping web developers understand JavaScript.

This single-threaded paradigm is inherently restrictive as it prevents programming patterns that are otherwise feasible in languages capable of delegating work to separate threads or processes. JavaScript is bound to this single-threaded paradigm to preserve compatibility with the various browser APIs that it must interact with. Constructs such as the Document Object Model would encounter problems if subjected to concurrent mutations via multiple JavaScript threads. Therefore, traditional concurrency constructs such as POSIX threads or Java's Thread class are non-starters for augmenting JavaScript.

Therein lies the core value proposition of workers: Allow the primary execution thread to delegate work to a separate entity without changing the existing single-threaded model. Although the various worker types covered in this chapter all have different forms and functions, they are unified in their separation from the primary JavaScript environment.