Typescript misunderstanding with ElasticSearch Node Client

Using:

• NodeJS v16.16.0
• "@elastic/elasticsearch": "8.7.0",

I want to create a function that will accept multiple kind of bulk operations.
The goal for me is to ensure the input of this functions will affect my index with good values (here a or b).

Here is the code snippet that i cannot correctly type:

import { Client } from "@elastic/elasticsearch";

// Operation 1 affect 1 index with "index" action
interface Op1IndexRow {
  index: {
    _index: string;
    _id: string;
    retry_on_conflict: number;
  }
}

interface Op1BulkDocument {
  a: string,
}

type Op1BulkBody = Array<Op1BulkDocument | Op1IndexRow>;


interface Op2IndexRow {
  index: {
    _index: string;
    _id: string;
    retry_on_conflict: number;
  }
}

interface Op2BulkDocument {
  b: string,
}

type Op2BulkBody = Array<Op2BulkDocument | Op2IndexRow>;

const client = new Client({
  node: 'localhost'
})

const bulkSave = async (operations: Op1BulkBody | Op2BulkBody) => {
  await client.bulk({
    refresh: true,
    operations,
  });
}

The thing I really don't understand is that i can make it "typescript friendly" with the following type: Array<Op1BulkDocument | Op1IndexRow | Op2BulkDocument | Op2IndexRow>.

The problem is that this type is wrong for me because the operations is either Op1BulkDocument | Op1IndexRow either Op2BulkDocument | Op2IndexRow but never both at the same time.

The code you provided almost does what you want. For example, it does not allow you to mix Op1BulkDocument with Op2BulkDocument:

declare const op1doc: Op1BulkDocument;
declare const op2doc: Op2BulkDocument;
bulkSave([op1doc, op2doc]);

> Argument of type '(Op1BulkDocument | Op2BulkDocument)[]' is not assignable to parameter of type 'Op1BulkBody | Op2BulkBody'.

That is because Op1BulkDocument has a member a which Op2BulkDocument does not have.

However it does allow you to mix Op1IndexRow with Op2IndexRow:

declare const op1index: Op1IndexRow;
declare const op2index: Op2IndexRow;
bulkSave([op1index, op2index]);

Why? The error message already gives you a hint: Op1IndexRow en Op2IndexRow have the same definition. You can assign a variable of type Op1IndexRow to a variable op type Op2IndexRow, because they have the compatible definitions.

TypeScript does not look at the exact interface name but checks if the interface definition is the compatible (is assignable).

I suggest you use a single interface OpIndexRow instead of two different (but the same) interfaces.