Array of union type generates incorrect syntax: `A | B | C[]` instead of `Array<A | B | C>`

[graham-macdonald-simplisafe]

Summary

When an Avro schema has an array whose items are a union type, avroToTypeScript generates incorrect TypeScript syntax. The [] suffix is appended after the last union member only, rather than wrapping the full union in Array<...>. The result is a type that TypeScript parses differently from what was intended.

Reproduction

Avro schema:

{
  "type": "record",
  "name": "Analysis",
  "fields": [
    {
      "name": "detections",
      "type": ["null", {
        "type": "array",
        "items": [
          { "type": "record", "name": "MotionDetection", "fields": [] },
          { "type": "record", "name": "PersonDetection", "fields": [] },
          { "type": "record", "name": "FaceDetection", "fields": [] }
        ]
      }],
      "default": null
    }
  ]
}

Generated output (incorrect):

export interface Analysis {
    detections: null | MotionDetection | PersonDetection | FaceDetection[];
}

Expected output:

export interface Analysis {
    detections: null | Array<MotionDetection | PersonDetection | FaceDetection>;
}

Why this is wrong

TypeScript parses null | MotionDetection | PersonDetection | FaceDetection[] as:

null  |  MotionDetection  |  PersonDetection  |  (FaceDetection[])

The array type applies only to FaceDetection, not to the full union. A detections value of type MotionDetection[] would be incorrectly accepted since MotionDetection (non-array) is a union member.

Root cause

In convertType, when isArrayType is true, the items type is converted recursively and [] is appended:

else if (isArrayType(type)) {
    return convertType(type.items, buffer, opts) + "[]";
}

When type.items is itself a union (an array in Avro schema), convertType returns a |-joined string and [] is appended to the last member only.

Fix

When items is a union (i.e. the result contains |), wrap in Array<...> instead of appending []:

else if (isArrayType(type)) {
    const itemType = convertType(type.items, buffer, opts);
    return itemType.includes(' | ') ? `Array<${itemType}>` : `${itemType}[]`;
}

Environment

• avro-typescript version: 1.3.0
• Node.js: 20.11.1