react functional component let variable doesn't changed

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I wrote functional component with react native.
but I’m in trouble because of a strange problem that seems to be a bug.

My purpose is changing variable in react functional component,
but variable not changed.

–I know It will be solved with change
let isDirty
to
cosnt [isDirty,setIsDirty] = useState(false)

Why doesn’t "isDirty" change when the handleTitleChange event occurs?

In pure js, it works very well

update.tsx

import React, { useState } from 'react';
import { View, TextInput, CheckBox, Button } from 'react-native';
import { NavigationInjectedProps } from 'react-navigation';
import TodoItem from '../models/todoItem';
import { updateItem } from '../DataProvider';

interface UpdateProps extends NavigationInjectedProps {
}

const Update: React.FC<UpdateProps> = (props) => {
  const [item, setItem] = useState(props.navigation.getParam("item") as TodoItem)
  let isDirty=false

  const handleSave=()=>{
    updateItem(item)
    
    if(isDirty&&props.navigation.state.params)
    {
      props.navigation.state.params.onUpdated(item);
    }
    props.navigation.goBack();
  }

  const handleCompleteChange= (value:boolean)=>{
    isDirty=true
    setItem({...item,isComplete:value})
  }

  const handleTitleChange=(txt:string)=>{
    isDirty=true
    setItem({...item,title:txt})
  }

  const handleContentChange=(txt:string)=>{
    isDirty=true
    setItem({...item,contents:txt})
  }
  
  return <View>
    <CheckBox value={item.isComplete} onValueChange={handleCompleteChange}></CheckBox>
    <TextInput defaultValue={item.title} onChangeText={handleTitleChange}></TextInput>
    <TextInput defaultValue={item.contents} onChangeText={handleContentChange}></TextInput>
    <Button title="Save" onPress={handleSave}></Button>
  </View>;
};

export default Update

pure js

const Update = () => {
  let isDirty=false;

  const handleTitleChange=(txt)=>{
    console.log(isDirty);
    isDirty=true;
    console.log(isDirty);
  }
  handleTitleChange("a");
  handleTitleChange("b");
};


Update();

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