In this chapter, we introduced some motivations for concurrency in JavaScript. While synchronous JavaScript is easy to maintain and understand, asynchronous JavaScript code is inevitable on the web. So it's important to make concurrency our default assumption when writing JavaScript applications.
There's two main types of concurrency we're interested in—asynchronous actions and parallel actions. Asynchronicity is about the time ordering of actions, which gives the impression that things are happening at the same time. Without this type of concurrency, the user experience would suffer greatly, because it would constantly be waiting on other actions to complete. Parallelism is another type of concurrency that solves a different type of problem, where we want to increase performance by computing results faster.
Finally, we looked at the three principles of concurrency in JavaScript programming. The parallelize principle is about leveraging the multi-core CPUs found in modern systems. The synchronize principle is about creating abstractions that enable us to write concurrent code, hiding the concurrency mechanisms from our feature code. The conserve principle uses lazy evaluation to only compute what is needed and to avoid unnecessary memory allocations.
In the next chapter, we'll turn our attention to the JavaScript execution environment. To be effective with JavaScript concurrency, we need a sound understanding of what's actually happening when our code is run.