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What a local NIF check can—and cannot—tell you
The Portuguese tax authority, Autoridade Tributária e Aduaneira (AT), describes an individual NIF as nine digits: the first eight are sequential and the last is a check digit. Its guidance says a person’s NIF remains the same whether they are registered as a resident or non-resident. AT guidance on the NIF structure.
A checksum function can detect many mistyped or incorrectly formed values. It cannot establish that a number was allocated to a taxpayer or remains active. Nor does it verify a business’s registration for intra-EU VAT. The European Commission’s separate TIN checker also warns that its online check does not confirm a person’s identity or whether the TIN exists: Commission TIN guidance.
Validate the nine-digit checksum in Node.js
The commonly documented Portuguese NIF checksum multiplies each of the first eight digits by weights from 9 down to 2, adds the results, and takes the sum modulo 11. If the remainder is 0 or 1, the check digit is 0; otherwise it is 11 minus the remainder. This formula is documented by technical references and implementations; the AT pages cited above confirm the existence of a check digit but do not publish this calculation.
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export function hasValidPortugueseNifChecksum(value) {
if (typeof value !== 'string' || !/^d{9}$/.test(value)) return false;
let sum = 0;
for (let i = 0; i < 8; i += 1) {
sum += Number(value[i]) * (9 - i);
}
const remainder = sum % 11;
const checkDigit = remainder < 2 ? 0 : 11 - remainder;
return Number(value[8]) === checkDigit;
}Call the function with a string, for example hasValidPortugueseNifChecksum('123456789'). The example illustrates the expected input shape; its result depends on the checksum and is not a claim that the number belongs to anyone. Keeping the NIF as a string avoids treating an identifier as a quantity and preserves any leading zero.
Define input normalization explicitly
The function intentionally accepts only exactly nine ASCII digits. It returns false for numbers, null, whitespace, prefixes, punctuation, or non-ASCII numeral characters. If your application needs to accept spaces or separators, remove only the specific characters your input contract allows, then run the strict check. Do not silently strip arbitrary characters: that can turn malformed or unintended input into a different identifier.
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Be cautious with prefix rules
Some implementations also restrict the first digit or prefix range. Such a whitelist encodes assignment policy and may change. The cited AT pages establish the nine-digit structure, but do not provide a complete current official prefix list. If your application needs prefix filtering, maintain it as separately sourced policy data rather than presenting it as an AT-endorsed rule.
Choose between a small function and a package
A direct function is appropriate when the requirement is limited to the documented checksum and your team is prepared to maintain tests and input rules. A package may be more convenient when you already use a validation library or need consistent handling of multiple identifier types. The existence of a package does not establish its current maintenance quality or suitability for a particular project.
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| Option | What the cited source establishes | What to check before adopting |
|---|---|---|
| Local checksum function | The checksum calculation is documented by a technical reference; the AT confirms that the NIF has a check digit. Technical NIF reference | Your tests, accepted input format, prefix policy, and any changes you choose to maintain. |
pt-id |
The npm page documents Portuguese identity-number validation, including a NIF validator; the reviewed registry page showed version 1.2.0. npm package page | Current release, license, tests, API, input normalization, TypeScript support, and maintenance status. |
| validator.js | Version 13.15.15 distributed source includes a pt-PT NIF check. validator.js source |
Whether that version or a current release fits your project, its license, tests, behavior, and release status. |
For either package, verify behavior against test cases you control rather than assuming that a dependency’s name or documentation makes it an official tax-authority validator.
Use VIES to check EU VAT registration
When the question is whether a business is registered for cross-border EU VAT, a local checksum is the wrong test. The European Commission describes VIES as a search engine that retrieves data from national VAT databases, not as a database of its own. A valid response means VAT information exists in the relevant national data; an invalid response means the number was not registered there at query time. European Commission VIES guidance (last checked 18 June 2026).
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An invalid result does not have just one explanation. The business may have no VAT number, may not be activated for intra-EU transactions, or may still be completing registration. National databases can also be temporarily unavailable. If VIES reports an error or service unavailability, retry rather than treating it as a definitive invalid result. Where tax control requires it, retain a record of a successful check.
VIES addresses VAT registration status; it is not a check of an individual’s identity. The Commission’s separate TIN guidance describes the scope and limits of checks for natural-person tax identification numbers: TIN guidance.
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Handle country prefixes and related identifiers separately
Do not make a validator accept or remove a country prefix without specifying the format your application expects. In Portuguese tax-authority webservice guidance, the issuer NIF field is specified without a country prefix. In a separate AT invoice manual, international customer identifiers use a country field. These are distinct integration contexts, so model the country and identifier according to the relevant interface rather than treating a prefixed value as interchangeable with a bare NIF.
Portugal’s EORI identifier is also distinct: for Portuguese operators it is formed as PT plus the Portuguese NIF. It is a customs identifier, not a different NIF checksum format to fold into this function. AT NIF guidance.
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