javascript - What are ES6 generators and how can I use them in node.js?

Question

I was at a node.js meetup today, and someone I met there said that node.js has es6 generators. He said that this is a huge improvement over callback style programming, and would change the node landscape. Iirc, he said something about call stack and exceptions.

I looked them up, but haven't really found any resource that explains them in a beginner-friendly way. What's a high-level overview of generators, and how are the different (or better?) than callbacks?

PS: It'd be really helpful if you could give a snippet of code to highlight the difference in common scenarios (making an http request or a db call).

Answer 1

Generators, fibers and coroutines

"Generators" (besides being "generators") are also the basic buildings blocks of "fibers" or "coroutines". With fibers, you can "pause" a function waiting for an async call to return, effectively avoiding to declare a callback function "on the spot" and creating a "closure". Say goodbye to callback hell.

Closures and try-catch

...he said something about call stack and exceptions

The problem with "closures" is that even if they "magically" keep the state of the local variables for the callback, a "closure" can not keep the call stack.

At the moment of callback, normally, the calling function has returned a long time ago, so any "catch" block on the calling function cannot catch exceptions in the async function itself or the callback. This presents a big problem. Because of this, you can not combine callbacks+closures with exception catching.

Wait.for

...and would change the node landscape

If you use generators to build a helper lib like Wait.for-ES6 (I'm the author), you can completely avoid the callback and the closure, and now "catch blocks" work as expected, and the code is straightforward.

It'd be really helpful if you could give a snippet of code to highlight the difference in common scenarios (making an http request or a db call).

Check Wait.for-ES6 examples, to see the same code with callbacks and with fibers based on generators.

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UPDATE 2021: All of this has been superseded by javascript/ES2020 async/await. My recommendation is to use Typescript and async/await (which is based on Promises also standardized)