为什么我的 useEffect 没有在第一次渲染时运行？

Question

So I am trying to scramble a sentence or string and push the words into an array to later join them and output this scrambled sentence but for some reason the useEffect is not running on the first render even with an empty array but when I change one line and run it again it runs as intended. I was just wondering if I was doing anything wrong and how I can change it so that it does run on the first render.

const ScrambleSentence = ({sentence}) => {
    const [words, setWords] = useState([]);
    const [scrambledSentence, setScrambledSentence] = useState('');
    let scrambledWord = [];


    function scrambleWord(n){
        var text = n.split('');
        for(var i = text.length - 1; i > 0; i--){
            var j = Math.floor(Math.random() * (i + 1));
            var temp = text[i];
            text[i] = text[j];
            text [j] = temp;
        }
        return text.join('');
    }

    useEffect(() => {
        setWords(sentence.split(' '));


        words.map(word =>{
          if(word.length <= 2){
            scrambledWord.push(word);

          }
          else{
            scrambledWord.push(scrambleWord(word));
          }
        })
        setScrambledSentence(scrambledWord.join(' '));
      }, [])


      console.log(scrambledSentence);
    return (
        <div>
            <p id='scrambled-word'>{scrambledSentence}</p>
        </div>
    )
};

export default ScrambleSentence;

Answer 1

Effects don't run during a render.

The first execution of an effect function will be triggered by the first render.

Answer 2

below code will not work as map need return statement, so it would be undefined, useEffect will console the empty array as define in the state. also you should not use let in render components as it will always initate, use may be useRef for that. also if your component consist js code, better to make it helper file and use it, would save and boost the performance.

 words.map(word =>{
              if(word.length <= 2){
                scrambledWord.push(word);
    
              }
              else{
                scrambledWord.push(scrambleWord(word));
              }
            })

Answer 3

useEffect is called after the first render however the issue in your code is that the words state isn't updated when you think it is.

useEffect(() => {
        setWords(sentence.split(' ')); 
// Here you set the words state that will trigger a new render. However the words variable is empty won't be changed until the next render


        words.map(word =>{ //Here you call map on an empty array.
          if(word.length <= 2){
            scrambledWord.push(word);

          }
          else{
            scrambledWord.push(scrambleWord(word));
          }
        })

        setScrambledSentence(scrambledWord.join(' ')); 
        //Here you set the state on an empty string
      }, [])
// the useEffect doesn't run a second time because it has no dependencies.

This code should work:

    const ScrambleSentence = ({sentence}) => {
    const [scrambledSentence, setScrambledSentence] = useState('');
    let scrambledWord = [];


    function scrambleWord(n){
        var text = n.split('');
        for(var i = text.length - 1; i > 0; i--){
            var j = Math.floor(Math.random() * (i + 1));
            var temp = text[i];
            text[i] = text[j];
            text [j] = temp;
        }
        return text.join('');
    }

    useEffect(() => {

        const words = sentence.split(' ')
        words.forEach(word =>{
          if(word.length <= 2){
            scrambledWord.push(word);
          }else{
            scrambledWord.push(scrambleWord(word));
          }
        })

        setScrambledSentence(scrambledWord.join(' '));
      }, [])


      console.log(scrambledSentence);
    return (
        <div>
            <p id='scrambled-word'>{scrambledSentence}</p>
        </div>
    )
};

export default ScrambleSentence;

https://reactjs.org/docs/hooks-state.html https://reactjs.org/docs/hooks-effect.html