在其他 function 中调用异步 function

Question

I am having a problem with asynchronous functions. I have the following function which works fine and basically searches into the firebase realtime database for a matching username:

  static async getSnapshot(fc: FormControl){
    let isPresent:boolean = false;
    await firebase.database().ref().child("users").orderByChild("username")
    .equalTo(fc.value)
    .once("value", snapshot => {          
    }).then((data)=> {
      if(data.exists())
        isPresent = true;
      else
        isPresent = false;
    });
    console.log(isPresent);
    return isPresent; 
  }

The problem is when I call this function in another where I want to do other operations based on the result:

  static async validUsername(fc: FormControl){
    try{
      let bool:boolean =await this.getSnapshot(fc.value)
      if(bool===true)
         return  ({validUsername: true});         
      else{
         return (null); 
       } 
      }catch(e){
        console.log(e)
      }         
  }   

The line:

let bool:boolean =await this.getSnapshot(fc.value)

returns the following error:

TypeError: Cannot read property 'getSnapshot' of undefined

How can I modify my function? Thanks in advance for response

Answer 1

this refers to an instance usually. Static methods don't belong to any instance so this doesn't make sense in them.

To fix your case, just use your class name instead of this . Eg

   class APIHandlers {
      static async getSnapshot {...}

      static async validUsername(fc: FormControl){
          try{
            let bool:boolean = await APIHandlers.getSnapshot(fc.value);
          ...
       } 
   }