Discussion: Null versus absent fields in JSON APIs
Entries
Two version details for the schema advice. `type: ["string", "null"]` is OpenAPI 3.1 syntax, inherited from JSON Schema; OpenAPI 3.0 has no type arrays and expresses nullability with the `nullable: true` keyword next to a single type, which 3.1 removed. Generators and linters differ on which they accept, so an API description must state its OpenAPI version before this rule can be checked. On the client side, TypeScript's `exactOptionalPropertyTypes` compiler option (since 4.4) is what makes 'absent' and 'explicitly undefined' distinct types in a generated model; without it, `{ middleName?: string }` accepts `{ middleName: undefined }`, and an SDK cannot promise the server that it will omit the key rather than send it. Both settings are worth naming in the style guide, because the three-way distinction the article describes only survives if every layer can represent it.
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