函数内部的全局变量不能在外部访问

Question

如果我理解正确，在函数内部不使用关键字 var 声明变量将创建一个全局范围的变量。但是当从其容器函数外部访问变量时，我得到这个“ReferenceError: oopsGlobal is not defined”。,,, // Declare the myGlobal variable below this linevar myGlobal = 10 function fun1() {  // Assign 5 to oopsGlobal Here  oopsGlobal = 5}// Only change code above this linefunction fun2() {  var output = "";  if (typeof myGlobal != "undefined") {    output += "myGlobal: " + myGlobal;  }  if (typeof oopsGlobal != "undefined") {    output += " oopsGlobal: " + oopsGlobal;  }  console.log(output);}console.log(oopsGlobal) // ReferenceError: oopsGlobal is not defined,,,

Answer 1

您编写了代码，但从未调用过它。fun1并且fun2永远不会运行。我在下面添加了一行，它调用了fun1()导致分配发生的函数。

这更像是提供演示的答案——您很可能不想实际编写具有全局变量或像这样的副作用的代码。如果您正在为浏览器编写软件，使用window或globalThis存储您的全局状态也可能使它更清晰。

// Declare the myGlobal variable below this line

var myGlobal = 10 

function fun1() {

  // Assign 5 to oopsGlobal Here

  oopsGlobal = 5

}

fun1(); // You wrote the functions previous, but you never CALLED them.

// Only change code above this line

function fun2() {

  var output = "";

  if (typeof myGlobal != "undefined") {

    output += "myGlobal: " + myGlobal;

  }

  if (typeof oopsGlobal != "undefined") {

    output += " oopsGlobal: " + oopsGlobal;

  }

  console.log(output);

}

console.log(oopsGlobal) // ReferenceError: oopsGlobal is not defined

Answer 2

发生这种情况是因为您实际上从未跑步过fun1()。如果你不调用一个函数，里面的代码将永远不会被执行。

参考错误：

 // Declare the myGlobal variable below this line

var myGlobal = 10 

function fun1() {

  // Assign 5 to oopsGlobal Here

  oopsGlobal = 5

}

// Only change code above this line

function fun2() {

  var output = "";

  if (typeof myGlobal != "undefined") {

    output += "myGlobal: " + myGlobal;

  }

  if (typeof oopsGlobal != "undefined") {

    output += " oopsGlobal: " + oopsGlobal;

  }

  console.log(output);

}

console.log(oopsGlobal) // ReferenceError: oopsGlobal is not defined

没有 ReferenceError（注意是之前fun1()调用的） console.log()

 // Declare the myGlobal variable below this line

var myGlobal = 10 

function fun1() {

  // Assign 5 to oopsGlobal Here

  oopsGlobal = 5

}

// Only change code above this line

function fun2() {

  var output = "";

  if (typeof myGlobal != "undefined") {

    output += "myGlobal: " + myGlobal;

  }

  if (typeof oopsGlobal != "undefined") {

    output += " oopsGlobal: " + oopsGlobal;

  }

  console.log(output);

}

fun1()

console.log(oopsGlobal)