Why your fake US test addresses fail validation (and how to fix it)

If you've ever seeded a staging database with faker and then watched
your checkout form reject every single address, this post is for you.

The problem: three layers of address validation

Address fields in a modern US checkout or signup form typically pass
through three layers:

1. Format checks — is the ZIP five digits, is the state a valid two-letter code? Any random data passes this.
2. Region-consistency checks — do the city, state and ZIP actually belong together? USPS-style validators and services like Google Address Validation all do this. 123 Main St, New York, CA 90001 dies here: New York is not in California, and 90001 is Los Angeles.
3. Deliverability checks — does the house number exist? Only logistics flows go this deep; test environments almost never need it.

The catch: most fake-data libraries generate each field
independently. faker.location.city() and
faker.location.zipCode() don't know about each other, so nearly every
generated record fails layer 2 — and your tests fail for reasons that
have nothing to do with your code.

Fix 1: hand-curated matched sets

For a handful of records, just keep a small table of combinations that
belong together:

Worth adding to any test suite as edge cases:

• Two-word states — Rhode Island, South Carolina: naive parsers split on spaces and read Rhode as a city.
• Washington, DC — not a state, but every form treats it as one. Its addresses carry quadrant suffixes (1600 K St NW ≠ 1600 K St NE).
• Utah grid addresses — 455 S 300 E has no street name at all. Great for breaking address parsers that assume Name + St/Ave.
• Tax-free states — OR, DE, MT, NH, AK have no statewide sales tax; pair them with a high-tax state like CA to test both checkout branches.

Fix 2: a dataset with the relationships baked in

I open-sourced the states/cities/ZIP building blocks I use (MIT):
us-address-sample-data
— 51 states with real area codes and tax flags, 255 cities where every
ZIP genuinely belongs to that city. Import the JSON and compose
addresses locally with zero dependencies.

Fix 3: an API that does it for you

For CI or quick scripts, I built a free endpoint (no key, CORS enabled,
up to 100 records per request):

bash
curl "https://addressmock.com/api/addresses?count=10&state=CA"
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const res = await fetch(
  "https://addressmock.com/api/addresses?count=20&type=us_tax_free"
);
const { results } = await res.json();
// every record: matched city/state/ZIP/area code + name, email, phone
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Parameters: count (1–100), type (us / us_tax_free / hk / cv), state, city, gender. There's also a browser UI with batch generation and CSV/JSON export if you prefer clicking to curling.

The obligatory disclaimer

All of this produces format-correct samples, not verified deliverable addresses. It's for testing, demos and seeding — using fake addresses for real shipping, KYC or dodging region checks is a different thing entirely, and platforms are good at catching it.

What other locales or formats would be useful? I'm considering UK postcodes and Canadian postal codes next.