This is my Api class. I initialize the variable client in init:

 export default class Api {
      constructor (options = {}) {   
        this.init()
      }
    
      async init () {
        this.config = await this.loadConfig()    
        this.client = this.initClient()     
        this.client.open()
      }
      initClient() {
           ...
      }
      loadConfig() {
      ...
      }
      fetchList() {
           console.log(this.client, this.config)
      }    
   }

Neither client nor config are accessible from the fetchList function. But they are initialized and have value when I console log them from the init function.

edit I call fetchList from react useEffect

useEffect(() => {
      let api = new Api()
      api.fetchTaskList({ type: 'active' })
      .then((data) => {
         console.log(data) 
      })
    },[])