Assertions in JavaScript

Assertions are a great way of catching errors during development. Since they’re inlined with your code they’re much cheaper to write than unit tests, while doing a better job of acting as documentation. I like to think of them as comments that actually enfore what they’re saying.

const value = values[i]!;
assert(value !== undefined, `values[${i}] must be defined`) 

Many languages have first party assertions, but JS doesn’t. The closest thing is the node:assert module, which isn’t available in browsers.

There are some additional complexities surrounding assertions with the JS ecosystem specifically.

1. Code is often bundle-size constrained. Adding assertions everywhere would bloat bundles.
2. JS projects often use a lot of libraries. Consumers of libraries should get the final say about what happens if an assertion in a library fails (fatal/log/ignore).

Ideally we would want assertions that:

1. Run in every JS environment
2. Can automatically be stripped from client side bundles to avoid bloat
3. Are non-fatal by default & can be made fatal by the final consumer

I’ve found that console.assert meets these criteria really well. It is universally supported, non-fatal by default, and easy for build tools to remove. Here is what I’ve been doing:

1. Add console.assert calls liberally througout my code
2. If I’m writing a library I do not strip out assertions before publishing
3. If I’m writing an app I use EsBuild’s drop option to removeconsole.assert calls from client builds. In vite this can be done via the esbuild option. I don’t remove assertions from the server-side build.
4. I monkey-patch console.assert to be fatal in server-side code.

So far this has been working well for me