Question

我正在尝试更新循环内循环之外的变量，如下所示：

var firstRun = true;
console.log("firstRun is " + firstRun);
if(firstRun == true){
    console.log("This is your first run");
    firstRun = false;
}else{
    console.log("You have already run this loop at least once");
};

假设这个代码块在一个较大的代码块中运行4次，我希望它输出This is your first run一次，然后You have already run this loop at least once三次。相反，我得到This is your first run 4次而console.log("firstRun is " + firstRun);总是输出true

我确定这是一个范围问题，我不太明白。原谅我，我来自Ruby的土地：）

Answer 1

您正在重新声明循环中的每一步firstRun。将var firstRun = true;移出循环。

如果你不能将它移出循环，你将不得不使用一个对象，并在分配对象之前检查它是否已被声明：

var firstRun = firstRun || {ran: false};

if (firstRun.ran == true) {
    console.log("This is your first run");
    firstRun.ran = false;
} else {
    console.log("You have already run this loop at least once");
};

Answer 2

我假设每次迭代都会重置布尔值。将var firstRun = true;移到循环外面！

var firstRun = true;
for (...) {
  if(firstRun){firstRun=false;}
  else{}
}

Answer 3

您每次都在循环中重置firstRun变量，因此当执行条件语句时，firstRun的值将始终为true。

你需要做这样的事情

var firstRun = true;

//Loop only following block
if(firstRun == true){
    console.log("This is your first run");
    firstRun = false;
}else{
    console.log("You have already run this loop at least once");
};

Answer 4

如果整个块在你的循环中，你将在每次迭代时重置var firstRun = true。你需要在循环之外设置var firstRun = true（为了清楚起见，在它之前）。

var firstRun = true;
for (var i = 0; i < 4; i++) {
    console.log("firstRun is " + firstRun);
    if(firstRun == true){
        console.log("This is your first run");
        firstRun = false;
    } else {
        console.log("You have already run this loop at least once");
    };
}

从循环内或条件中更新变量

4 个答案: