Lazy evaluation is a programming technique, which is used when we don't want to compute values until the very last second. This way, we're sure we actually need it. The opposite approach, eager evaluation, has the potential to compute several values that aren't needed. This generally isn't a problem, until the size and complexity of our applications grow beyond a level where these wasted computations are imperceptible to the user.
The Generator is a new primitive type introduced to JavaScript as a part of the ES6 specification of the language. Generators help us implement lazy evaluation techniques in our code, and as a corollary, help us implement the conserve concurrency principle.
We'll start the chapter off with some simple introductions to generators, so we can get a feel for how they behave. From there, we'll move onto more advanced lazy evaluation scenarios, wrapping up the chapter with a look at coroutines. Let's get started.