为什么在运行时返回未定义？

Question

function switchBooks(book) {
  book = "the book name";
}

var myBook = "new book";
myBook = switchBooks(myBook);

console.log(myBook)

我是 JS 新手，想了解为什么会这样？

Answer 1

进一步补充 UnholySheep 的评论。您需要 return 函数的结果以便稍后在您的代码中访问它。

来自MDN Web Docs

The return statement ends function execution and specifies a value to be returned to the function caller.

在你的例子中，函数调用者是 switchBooks(myBook);

   function switchBooks(book) {
       var book = "the book name";
       return book; //This is the addition
   }

   var myBook = "new book";
   myBook = switchBooks(myBook);

   console.log(myBook)

Answer 2

您需要在函数内部添加return。

function switchBooks(book) {
 return book = "the book name";
}

var myBook = "new book";
myBook = switchBooks(myBook);

console.log(myBook)

为什么在运行时返回未定义？

Why is this returning undefined when run?

javascript

function

console.log

为什么在 运行 时返回未定义？

Why is this returning undefined when run?

javascript

function

console.log

为什么在运行时返回未定义？