const params = {
   factor: 2
}
const field = esb.runtimeField("long", "emit(doc['measure'].value * params.factor);")
  .lang('painless')
  .params(params)
const reqBody = esb.requestBodySearch();
reqBody.runtimeMapping('measure', field);

{
  "runtime_mappings": {
    "measure": {
      "type": "long",
      "script": {
        "lang": "painless",
        "source": "emit(doc['measure'].value * params.factor);",
        "params": {
          "factor": 2
        }
      }
    }
  }
}