什么是 JavaScript 中的提升 | 提升函数、变量和类

矫情吗；* 2023-09-26 23:52 143阅读 0赞

提升是 JavaScript 中的一个概念或行为，其中函数、变量或类的声明到达它们定义的范围的顶部。这是什么意思？

提升是一个概念，您可能会在某些编程语言（例如 JavaScript）中找到这个概念，而在其他语言中则没有。这是 JavaScript 解释器的一种特殊行为。我们将在本文中了解它的工作原理。

让我们从功能开始。

我有这个主题的视频版本，您可以查看。

函数提升  
看看这个代码示例：

function printHello() \{  
console.log(“hello”)  
\}

printHello()  
// hello  
在这里，我们声明printHello，并在声明的行之后执行该函数。没有错误；一切正常！

现在看这个例子：

printHello()  
// hello

function printHello() \{  
console.log(“hello”)  
\}  
在这里，我们printHello在声明函数的行之前执行。一切仍然正常，没有错误。这里发生了什么？吊装。

Before the interpreter executes the whole code, it first hoists (raises, or lifts) the declared function to the top of the scope it is defined in. In this case, printHello is defined in the global scope, so the function is hoisted to the top of the global scope. Through hoisting, the function (including the logic) becomes accessible even before the line it was declared in the code.

Let’s see another example:

printHello()  
// hello

printDillion()  
// ReferenceError: printDillion is not defined

function printHello() \{  
console.log(‘hello’)

function printDillion() \{  
console.log(“dillion”)  
\}  
\}  
As you see here, we declare printHello. In this function, we first execute console.log(‘hello’) then we declare another function called printDillion which executes console.log(‘dillion’) when called.

Before printHello is declared in the code, we try to access it by executing printHello(). It’s accessible (since it is hoisted to the top of the global scope), so we have “hello” printed on the console.

But then we try to access printDillion,