Conditionals

Conditional schemas infer as discriminated unions narrowed on the condition field. @jetio/schema-builder provides type inference for JSON Schema’s conditional keywords — including elseIf, which no other TypeScript schema library supports.

The type is built by merging the base properties, if, and then, then forming an exclusive-or with the merge of base and else — so only one branch can exist at a time. This extends through every elseIf.

Basic if / then / else


const addressSchema = new SchemaBuilder()
  .object()
  .properties({
    country: (s) => s.string(),
    state: (s) => s.string(),
    postalCode: (s) => s.string(),
  })
  .required(["country"])
  .if((s) => s.object().properties({ country: (s) => s.const("US") }))
  .then((s) =>
    s
      .object()
      .properties({
        state: (s) => s.string(),
        zipCode: (s) => s.string().pattern("^[0-9]{5}$"),
      })
      .required(["state"]),
  )
  .else((s) =>
    s.object().properties({ postalCode: (s) => s.string() }).required(["postalCode"]),
  )
  .build();
 
type Address = Jet.Infer<typeof addressSchema>;
// {
//   country: "US"; state: string; zipCode?: string; postalCode?: string;
// } | {
//   country: string; postalCode: string; state?: string; zipCode?: undefined;
// }

if / elseIf / else chains


const shippingSchema = new SchemaBuilder()
  .object()
  .properties({
    country: (s) => s.string(),
    weight: (s) => s.number(),
    shippingMethod: (s) => s.string(),
  })
  .required(["country", "weight"])
  .if((s) => s.object().properties({ country: (s) => s.const("US") }))
  .then((s) =>
    s
      .object()
      .properties({
        shippingMethod: (s) => s.enum(["USPS", "UPS", "FedEx"]),
        estimatedDays: (s) => s.enum([2, 3, 5]),
      })
      .required(["shippingMethod"]),
  )
  .elseIf((s) => s.object().properties({ country: (s) => s.const("CA") }))
  .then((s) =>
    s
      .object()
      .properties({
        shippingMethod: (s) => s.enum(["Canada Post", "Purolator"]),
        estimatedDays: (s) => s.enum([5, 7, 10]),
      })
      .required(["shippingMethod"]),
  )
  .else((s) =>
    s
      .object()
      .properties({
        shippingMethod: (s) => s.string(),
        estimatedDays: (s) => s.number().minimum(14),
      })
      .required(["shippingMethod"]),
  )
  .build();
 
type Shipping = Jet.Infer<typeof shippingSchema>;
// {
//   country: "US"; weight: number;
//   shippingMethod: "USPS" | "UPS" | "FedEx"; estimatedDays?: 2 | 3 | 5;
// } | {
//   country: "CA"; weight: number;
//   shippingMethod: "Canada Post" | "Purolator"; estimatedDays?: 5 | 7 | 10;
// } | {
//   country: string; weight: number;
//   shippingMethod: string; estimatedDays?: number;
// }

Key behaviours: conditionals create discriminated unions narrowed on the condition field (one is enforced if absent); elseIf is evaluated in order, first match wins; each branch is exclusive — data matches exactly one path.