How can I add an event handler to my dropdown so that <onChange> of the dropdown value, all the value representations on the screen changes?

UI:

I have a dropdown list with four types of currencies. Every time I pick a new currency, the currency representations on the screen should also change. When I choose the first time, let's say Euro, all my currency marks go blank. Then, if I pick Pound, all my currency marks a change to the Euro sign. Why my state change is delayed?

components/currency.js

import React, { useState,useContext,useEffect } from 'react';import { AppContext } from '../context/AppContext';

const Budget = () => {const [currency, setCurrency] = useState('');const { dispatch } = useContext(AppContext);

const handleChange = (event) => {
    setCurrency(event.target.value);
    changeCurrency()
  };

  const changeCurrency = () => {
    dispatch({
        type: 'CHG_CURRENCY',
        payload: currency,
    });
  }

useEffect(() => {console.log(currency);}, [currency] )

return (
    <div className="alert alert-secondary">
    <select id="inputGroupSelect02" onChange={handleChange}>
            <option defaultValue>Currency</option>
            <option value="$" name="dollar">$ Dollar</option>
            <option value="£" name="pound">£ Pound</option>
            <option value="€" name="euro">€ Euro</option>
            <option value="₹" name="ruppee">₹ Ruppee</option>
              </select>
              </div>
);

export default Budget;

context/AppContext.js

import React, { createContext, useReducer } from 'react';

// 5. The reducer - this is used to update the state, based on the action
export const AppReducer = (state, action) => {
    let budget = 0;
    switch (action.type) {
        case 'ADD_EXPENSE':
            let total_budget = 0;
            total_budget = state.expenses.reduce(
                (previousExp, currentExp) => {
                    return previousExp + currentExp.cost
                },0
            );
            total_budget = total_budget + action.payload.cost;
            action.type = "DONE";
            if(total_budget <= state.budget) {
                total_budget = 0;
                state.expenses.map((currentExp)=> {
                    if(currentExp.name === action.payload.name) {
                        currentExp.cost = action.payload.cost + currentExp.cost;
                    }
                    return currentExp
                });
                return {
                    ...state,
                };
            } else {
                alert("Cannot increase the allocation! Out of funds");
                return {
                    ...state
                }
            }
            case 'RED_EXPENSE':
                const red_expenses = state.expenses.map((currentExp)=> {
                    if (currentExp.name === action.payload.name && currentExp.cost - action.payload.cost >= 0) {
                        currentExp.cost =  currentExp.cost - action.payload.cost;
                        budget = state.budget + action.payload.cost
                    }
                    return currentExp
                })
                action.type = "DONE";
                return {
                    ...state,
                    expenses: [...red_expenses],
                };
            case 'DELETE_EXPENSE':
            action.type = "DONE";
            state.expenses.map((currentExp)=> {
                if (currentExp.name === action.payload) {
                    budget = state.budget + currentExp.cost
                    currentExp.cost =  0;
                }
                return currentExp
            })
            action.type = "DONE";
            return {
                ...state,
                budget
            };
        case 'SET_BUDGET':
            action.type = "DONE";
            state.budget = action.payload;

            return {
                ...state,
            };
        case 'CHG_CURRENCY':
            action.type = "DONE";
            state.currency = action.payload;
            return {
                ...state
            }

        default:
            return state;
    }
};

// 1. Sets the initial state when the app loads
const initialState = {
    budget: 2000,
    expenses: [
        { id: "Marketing", name: 'Marketing', cost: 50 },
        { id: "Finance", name: 'Finance', cost: 300 },
        { id: "Sales", name: 'Sales', cost: 70 },
        { id: "Human Resource", name: 'Human Resource', cost: 40 },
        { id: "IT", name: 'IT', cost: 500 },
    ],
    currency: '£'
};

// 2. Creates the context this is the thing our components import and use to get the state
export const AppContext = createContext();

// 3. Provider component - wraps the components we want to give access to the state
// Accepts the children, which are the nested(wrapped) components
export const AppProvider = (props) => {
    // 4. Sets up the app state. takes a reducer, and an initial state
    const [state, dispatch] = useReducer(AppReducer, initialState);
    let remaining = 0;

    if (state.expenses) {
            const totalExpenses = state.expenses.reduce((total, item) => {
            return (total = total + item.cost);
        }, 0);
        remaining = state.budget - totalExpenses;
    }

    return (
        <AppContext.Provider
            value={{
                expenses: state.expenses,
                budget: state.budget,
                remaining: remaining,
                dispatch,
                currency: state.currency
            }}
        >
            {props.children}
        </AppContext.Provider>
    );
};

Spent so far component

import React, { useContext } from 'react';
import { AppContext } from '../context/AppContext';

const ExpenseTotal = () => {
    const { expenses,currency } = useContext(AppContext);

    const totalExpenses = expenses.reduce((total, item) => {
        return (total += item.cost);
    }, 0);

    return (
        <div className='alert alert-primary'>
            <span>Spent so far: {currency}{totalExpenses}</span>
        </div>
    );
};

export default ExpenseTotal;

Put the changeCurrency() in your useEffectHook.

This is happening because when you call setCurrency to update your state it does not update it instantly, instead it throws it in a queue which will not take effect until you current running stack gets empty. As a result you don't get the currency state updated instantly.

And I would recommend you to take look at this react documentation Which will describe you why this is happening in details

React state as a snapshot