javascript - 为什么分配给变量的函数直接放置在自调用匿名函数之上时会执行？

Question

运行以下代码时，我遇到了奇怪的结果:

var saySomethingElse, v;

// This function will not run when the nameless function runs, even if v and saySomethingElse are commented out.
function saySomething() {

  alert("something");

}

// When v is uncommented, this function will run when the nameless function below runs..
saySomethingElse = function() {

  alert("something else");

}

//v = "by uncommenting me, saySomethingElse will no longer be called.";

(function() {

  if (v) {

    alert("Now things are working normally.")

  }

  alert("This alert doesn't happen if v is commented out.");

})();

当此代码运行时，底部的匿名函数调用 saySomethingElse 而不是它自己的内容，但如果 v 未注释，一切都会按预期工作: saySomethingElse 不执行，匿名函数执行自己的内容。我认为这可能是正常行为，但我正在寻找解释。有谁知道为什么会发生这种情况？

查看 fiddle :working example

Answer 1

您需要在匿名函数末尾添加一个分号saySomethingElse

您应该始终以分号正确结束匿名函数。不需要使用分号来结束普通的非匿名 function fooBar() {} 函数。

var saySomethingElse, v;

// This function will not run when the nameless function runs, even if v and saySomethingElse are commented out.
function saySomething() {

  alert("something");

} // <-- Semi-colon not necessary.

// When v is uncommented, this function will run when the nameless function below runs..
saySomethingElse = function() {

  alert("something else");

};  // <-- Semi-colon recommended to prevent errors like you're getting.

//v = "by uncommenting me, saySomethingElse will no longer be called.";

(function() {

  if (v) {

    alert("Now things are working normally.")

  }

  alert("This alert doesn't happen if v is commented out.");

})();

由于 saySomethingElse 已在最后正确终止，代码现在可以按照您的预期运行。

这是因为，在 JavaScript 中，您应该在每个语句的末尾使用分号。匿名函数定义是语句，就像任何其他变量定义一样。