Callback function definition and Calling issue in react native

Question

Friend I will take one class for define to all function. Now We required some of the function with its callback. So I define as below.

CallBack define :

export const getStoredData(key: string, callback?: ?(error: ?Error, result: ?string)) =>{
      try {
      const value = await AsyncStorage.getItem(key);
      if (value !== null){
        return value
      }
    } catch (error) {
      return error
    }
}

Call the function as below :

   getStoredData('apple' , (error , result) =>{
     if (error) {
       console.log('error is = ', error);
     } else {
       console.log('result is = ', result);
     }
   });

But i have error to define function unexpected token.

Please help me.

Answer 1

As @MayankShukla said, you aren't using arrow function correctly.

To fix your expression, you also need to add =>void as below to fix the function type of callback,

export function getStoredData(key: string, callback?: ?(error: ?Error, result: ?string) => void ) {

In my vs code Unexpected Identifier disappeared with above code.

Edited for comment 1: for getting result from callback, try change the function

export async function getStoredData(key: string, callback?: ?(error: ?Error, result: ?string) => void ) {
    try {
        const value = await AsyncStorage.getItem(key);
        if (value !== null) {
            callback(null, value);
            return value;
        }
    } catch (error) {
        callback(error, null);
        return error;
    }
}

Answer 2

Your syntax is wrong because you're not exporting a function as you may want. Try assigning your constant and close the brackets properly :

export const getStoredData = (key, callback) => {
  try {
  const value = await AsyncStorage.getItem(key);
  if (value !== null){
    return value
  }
  } catch (error) {
    return error
  }
};

getStoredData('apple', (error, result) => {
 //do stuff
});

Answer 3

You are using arrow function in a wrong way, you forgot =, Unexpected token is because of this => here:

export const getStoredData(key: string, callback?: ?(error: ?Error, result: ?string)) => {
   ....
}

Either write it like this (note = after getStoredData):

export const getStoredData = (key: string, callback?: ?(error: ?Error, result: ?string)) => { 
    ....
}

or remove the =>

export function getStoredData (key: string, callback?: ?(error: ?Error, result: ?string)){
   ....
}

Answer 4

/*

* created by Suresh Mewara

* Date 09-07-2018

*/

import React, {Component} from 'react';
import {AsyncStorage, } from 'react-native';

export class DBPreference {
  // Database key
  static LOGIN_STATUS = 'loginStatus';
  static EULA_STATUS = 'eulaStatus';
  static ACCESS_CODE = 'accessCode';





  static retrieveData = async (key) => {
    try {
      const value = await AsyncStorage.getItem(key);
      if (value !== null) {
        // We have data!!
        console.warn(value);
        return value;
      }
     } catch (error) {
       // Error retrieving data
     }
  }



  static getStoredData = async (key, callback) => {
  try {
  const value = await AsyncStorage.getItem(key);
  if (value !== null){
    callback(null, value);
    console.warn(value);
    return value
  }
  } catch (error) {
    callback(error, null);
    return error
  }
};
}