Async/await vs Coroutines vs Promises vs Callbacks

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How are async/await, coroutines, promises and callbacks related? Which one is better or worse? We are going to implement the same code with these 4 different approaches.

First of all, let’s define a simple delay function, that resolves with a given value after a given amount of time elapsed.

Async Code With Async/await in JavaScript

This is the most recent feature of all 4 approaches.

Pros

• Clean code, low syntax noise
• Native in current Chrome, Node.js

Cons

• Still needs to be transpiled with Babel for most projects

Async Code With Promise Chaining

This is more oldschool than async/await.

Pros

• No extra libraries needed since Promises are native

Cons

• Noisy callbacks

Async Code With Callbacks

Years ago this was the usual way to handle async continuations.

Pros

• Easy to understand
• No libs needed, backward compatible

Cons

• Pyramid of doom
• More difficult to maintain

Async Code With Coroutines

This is the most exotic one, having similar syntax to async/await

Pros

• No pyramid of doom
• Flow can be traced easily

Cons

• Some coroutine library is needed
• Generators must be used

If you are curious how a simple coroutine runner works, here we go. It’s just a simplified implementation to have an understanding of the concept.

Conclusions & Opinions

In most cases async/await is the best choice, since it makes the code maintainable and it’s supported by Node.js and modern browsers. Older browsers or Node.js versions can be targeted by Babel transpiler.

Some folks like to write callback hells and pyramid of dooms. It’s time for them to move on and practise coding withasync/await.

Of course, without a deep knowledge of Promises, async/await or coroutines are just magic, so it is very valuable to have a close friendship with Promises.

A typical JavaScript interview question could be to write equivalent code with the 4 above approaches.

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