Don't allow a schema to be nullable more than once

Author: ericallam

src/jsonSchema/index.ts

@@ -8,7 +8,11 @@ export function toJSONSchema(inferredSchema: InferredSchema): SchemaBuilder<Sche
     .with({ type: "unknown" }, () => s.$false()) // This should never be reached
     .with({ type: "boolean" }, () => s.boolean())
     .with({ type: "nullable" }, ({ schema }) =>
-      schema.type == "unknown" ? s.nil() : s.nullable(toJSONSchema(schema)),
+      schema.type == "unknown"
+        ? s.nil()
+        : schema.type === "nullable"
+        ? toJSONSchema(schema)
+        : s.nullable(toJSONSchema(schema)),
     )
     .with({ type: "int" }, () => {
       return s.integer();

tests/jsonSchema.test.ts

@@ -339,6 +339,17 @@ describe("nullables", () => {
       },
     });
   });
+
+  test("infers just a single null type if multiple nulls are found", () => {
+    const schema = inferSchema([1, 2, null, 3, null, 4]);
+
+    expect(toSchema(schema)).toStrictEqual({
+      type: "array",
+      items: {
+        type: ["integer", "null"],
+      },
+    });
+  });
 });
 
 describe("real world tests", () => {