Using Facebook's invariant vs if throw

Question

I've been looking at various Node.js projects' source, and I've noticed that some people use invariant. From what I understood, invariant is a tool that lets you put assertions in your code, and raise errors as needed.

Question:

When would you favor using invariant vs throwing errors the traditional way?

// Using invariant
function doSomething(a, b) {
   invariant(a > b, 'A should be greater than B');
}

// If throw
function doSomething(a, b) {
   if(a <= b) {
      throw new Error('A should be greater than B');
   }
}

Answer 1

zertosh/invariant allows to add code guards

As said in the readme it is A way to provide descriptive errors in development but generic errors in production.

however it is a replication of some internal facebook's systems and imo is pretty bad documented and not maintained. Scary thing is the 4.4M uses :thinking:

• nothing will be striped out of the box
• if you don't have a build tool that somehow remove your message in production you will still have the original error
• the usage in node is for ssr/react native, or useless outside of the "we have less lines" thing
• it uses error.framesToPop which also is a facebook thing

see: https://github.com/zertosh/invariant/issues?q=is%3Aissue

Note: A better aproach will be to wait for the es proposal throw inline and actually do

cond || throw x
cond ?? throw x

that way the error will not be evaluated anyway and stripped if cond includes a falsy var env in the browser

Answer 2

Usefulness in TypeScript projects

...Adding on to previous answers of making it easier to read, less lines of code, stripping from dev builds:

If you're using typescript, you can use it help narrow down types + get dev time feedback.

Imagine the scenario below: We're reading from our filesystem in node/js, the type system has no idea what's in there, so we need a runtime check, for that we'll want an invariant method to make runtime checks like this easy.

Note:

• there is a modern & popular version Facebook's invariant package called tiny-invariant which I recommend called tiny-variant: https://github.com/alexreardon/tiny-invariant