How Firebase functions.ignore really works

April 7, 2023

Maybe you ran into Firebase functions space issues, which is not uncommon if you’re moving Firebase functions inside a monorepo. Anyhow, you’re now playing with the functions.ignore list in your firebase.json.

The ignore list is not well documented, nor is the whole firebase.json really, but there is a section on it in the Firebase CLI reference. I always struggle to find this page, and I systematically find it through this GitHub issue about documenting it.

It says the following about the ignore list:

The list of files ignored by default, shown in JSON format, is:

  "ignore": [

If you add your own custom values for ignore in firebase.json, make sure that you keep (or add, if it is missing) the list of files shown above.

Let’s dig into it in a bit more details.

If you set an ignore list, it overrides the defaults!

That’s the only thing the docs say about the ignore list. It’s important to note because otherwise, you may notice that setting "ignore": [] ends up including much more stuff than not setting it, and this can be surprising.

This is actually partially true (but mostly true to be fair).

We can see in the source code the following:

const ignore = config.ignore || ['node_modules', '.git']


This means that regardless if you customize or not functions.ignore, the debug logs and runtime config will always be ignored. But it’s also true that if you explicitly set functions.ignore and forget to add node_modules and .git, those will indeed be included. Now you know.

You can’t use / to refer to the functions root

In .gitignore and any sane ignore format, you can use / to refer to the project root. E.g. ignoring /bar will ignore bar at the top level, but will still include foo/bar.

This is pretty handy in a number of situations, and just a good practice in general to be more intentional about what you mean to exclude. If you have a data directory that you want to ignore, but you just put data in your ignore file, and later on you add src/api/data/load.js, guess what, src/api/data will be ignored and you’ll be confused until you figure out the sneaky ignore pattern. You really should be ignoring /data in that case.

So again, we can’t do that in functions.ignore. Why? Ultimately, this is because Firebase uses minimatch for this here, but they pass the system-wide absolute path as the first argument! So that’s what ends up happening when using a / pattern (using matchBase and dot to mimic how Firebase uses it):

> minimatch('/path/to/functions/foo', '/foo', { matchBase: true, dot: true })

What would be great is if the first argument was “scoped” to the functions root. Then we would have nice things:

> minimatch('/foo', '/foo', { matchBase: true, dot: true })

But because we can’t have nice things, we have to resort to using a wider pattern (without the /):

> minimatch('/path/to/functions/foo', 'foo', { matchBase: true, dot: true })

This is a problem though because as we saw earlier, it’s too wide:

> minimatch('/path/to/functions/src/foo', 'foo', { matchBase: true, dot: true })

Can’t use / in base patterns

The last example works only because matchBase is enabled, but if you look at the matchBase documentation, you see that it breaks down as soon as our pattern includes a /:

> minimatch('/path/to/functions/foo/bar', 'foo/bar', { matchBase: true, dot: true })

But since root patterns don’t work anyway as we just saw, we can get around this by using wildcards:

> minimatch('/path/to/functions/foo/bar', '**/foo/bar', { matchBase: true, dot: true })

There’s no pattern negation

Pattern negation is what allows you do do something like this in a .gitignore:


This would ignore everything in the dist directory except for dist/package.json.

This is particularly handy in a number of situations, especially when you consider the alternative which is to explicitly ignore every single file or directory you have in dist. And obviously, remembering to add any new file to your ignore list when you create them, or when tools you use create other random files you don’t even know exist.

All that to say, you guessed it, that Firebase functions.ignore doesn’t support this.


That’s all I have for today. If you’re working with Firebase functions.ignore right now and noticed a few quirks, I hope this made it easier for you to understand what’s going on.

And if you’re trying to fix a Firebase functions source being too large to be deployed, I also wrote a post with tips to troubleshoot it.

Peace. ✌️

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