Reading the source beats reading the docs

I lost most of a day last week to a caching bug. The docs for the library said the cache key was derived from the request URL. That was true, but incomplete: it also folded in a header I was setting dynamically, so my cache hit rate was effectively zero.

None of this was written down. All of it was thirty lines into a file I could have opened at any point.

The habit worth building

When a library behaves in a way you didn't predict, open node_modules before you open a search engine.

You already have the exact version you're running, which is more than any blog post or Stack Overflow answer can promise you.

// The fastest way to find where you actually are
console.log(require.resolve('the-library/package.json'));

From there, d.ts files are usually the highest signal-per-line reading in the whole package. They're the author's own summary of what the thing does.

What you get out of it

1. Version-accurate answers. No guessing whether the post you're reading predates a rewrite.
2. The edge cases. Docs describe the happy path because that's what docs are for.
3. Better bug reports. "This throws at cache.ts:112 when headers is undefined" gets fixed a lot faster than "caching seems broken."

It's not a substitute for good documentation. It's just that the source is always there, always current, and nobody forgot to update it.