在ElasticSearch中，文档有一些 meta信息，也有文档的来源，所以文档结构如下：

{
   "_index": "index-name",
   "_id": "document-id-stored-by-elasticsearch",
   "_source": {
      // documents itself
   }
}

因此，在文档内部，您不能使用_id字段，如下所示，因为_id字段是保留字段：

POST myindex/_doc
{
  "_id": "123123",
  "field1": "value1"
}

因此，此请求将给予如下错误：

{
  "error": {
    "root_cause": [
      {
        "type": "mapper_parsing_exception",
        "reason": "Field [_id] is a metadata field and cannot be added inside a document. Use the index API request parameters."
      }
    ],
    "type": "mapper_parsing_exception",
    "reason": "failed to parse field [_id] of type [_id] in document with id '16Y6koUBnxYxC21BP-tS'. Preview of field's value: '123123'",
    "caused_by": {
      "type": "mapper_parsing_exception",
      "reason": "Field [_id] is a metadata field and cannot be added inside a document. Use the index API request parameters."
    }
  },
  "status": 400
}

在您的示例中，doc变量（其值来自Mongodb）有一个字段_id。您需要将该字段的名称更改为id。您可以将以下行放在flatMap函数中的匿名函数中。

db.collection("article_beta")
  .find()
  .limit(100)
  .toArray((err, docs) => {
    if (err) throw err;

    esClient.bulk(
      {
        body: docs.flatMap((doc) => {
          doc.id = doc._id  // <---- added
          delete doc._id    // <---- added
          return  [
            {
              index: {
                _index: "impact-fulltext",
                _id: doc.id.$oid,
              },
            },
            doc,
          ]
        }),
      },
      (err, resp) => {
        if (err) throw err;
        console.log(resp);
        client.close();
      }
    );
  });