javascript - 这是 JS array.find 的错误使用还是需要忽略的 VSCode 警告？

Question

我需要用另一个对象替换数组中的特定对象。我使用的方法是在原始数组上使用 find ，然后覆盖找到的值。据我了解这会改变原始数组。该方法似乎成功实现了我的要求，但我在 VScode 中收到“变量从未读取”警告。

屏幕截图

代码

for (let newQuestionObj of questionWIthNumericCorrectNumericValue) {
    let requiredOriginalQuestion = allPotentialAnswers.find(originalQuestion => {
        return originalQuestion.originalQ === newQuestionObj.originalQ;
    });

    //change question to new value
    requiredOriginalQuestion = newQuestionObj;
}

这种方法有什么问题吗？或者我应该忽略这个警告吗？

Answer 1

这里的问题是你的范围。让我们一步一步来:

for (let newQuestionObj of questionWIthNumericCorrectNumericValue) {
    // defining the variable
    let requiredOriginalQuestion = allPotentialAnswers.find(originalQuestion => {
        return originalQuestion.originalQ === newQuestionObj.originalQ;
    });
    //useless find.... because you are going to kill the result of the find.

    // reassign the variable to a new thing
    requiredOriginalQuestion = newQuestionObj;
} // < -- here, requiredOriginalQuestion DIES.

所以基本上，以更简单的方式发生的事情是这样的

for (let something from someArray){
    let newvar = doExpensiveCalculation();
    newvar = "--";
    // and we didnt do anything with it, despite of reassigning to "--"
} <-- newvar doesnt exist anymore.

或更明确地说:

for (let something from someArray){
    // lets keep doing nothing :D
}

所以基本上你的警告是这样的

requiredOriginalQuestion is declared but its value is never read.

发生这种情况是因为在每个循环结束时，您没有对 let requiredOriginalQuestion 执行任何操作。您也可以检查它是否会消失，执行以下操作:

for (let newQuestionObj of questionWIthNumericCorrectNumericValue) {
    let requiredOriginalQuestion = allPotentialAnswers.find(originalQuestion => {
        return originalQuestion.originalQ === newQuestionObj.originalQ;
    });
    requiredOriginalQuestion = newQuestionObj;

    const stringified = JSON.stringify(requiredOriginalQuestion);
} 

但是你会得到类似的东西:

stringified is declared but its value is never read.

终于

如果您想使用查找结果的响应覆盖该值，这就是您所需要的:

for (let newQuestionObj of questionWIthNumericCorrectNumericValue) {
    let requiredOriginalQuestion = allPotentialAnswers.find(originalQuestion => {
        return originalQuestion.originalQ === newQuestionObj.originalQ;
    });
    newQuestionObj = requiredOriginalQuestion;
} 

这样你就不会收到任何警告;)