コード

const params = {
  user_id: firebase.auth().currentUser.uid,
  id_token: firebase.auth().currentUser.getIdToken(false), // await が必要
}

const body = JSON.stringify(params) // エラー発生

エラー

TypeError: Converting circular structure to JSON
    --> starting at object with constructor 'C'
    |     property 'c' -> object with constructor 'C'
    |     property 'b' -> object with constructor 'zc'
    --- property 'a' closes the circle
    at JSON.stringify (<anonymous>)
    at _callee4$ (common.js?4edf:57)
    at tryCatch (runtime.js?96cf:63)
    at Generator.invoke [as _invoke] (runtime.js?96cf:293)
    at Generator.eval [as next] (runtime.js?96cf:118)
    at asyncGeneratorStep (asyncToGenerator.js?1da1:3)
    at _next (asyncToGenerator.js?1da1:25)
    at eval (asyncToGenerator.js?1da1:32)
    at new Promise (<anonymous>)
    at Object.eval (asyncToGenerator.js?1da1:21)

原因

awaitの付け忘れ。

解決コード

const params = {
  user_id: firebase.auth().currentUser.uid,
  id_token: await firebase.auth().currentUser.getIdToken(false), // awaitをつけた
}

const body = JSON.stringify(params)