函数中的 var 给出 undefined [duplicate]-解网

问：

JS吊装如何在函数中工作？（4 个答案）

26天前关闭。

更新 1：为什么我变得不明确

我正在尝试赚取js吊装。当我把它放在函数中时，我变得未定义，但是当我不放置任何函数时，我得到了输出。当我使用 let I are error 在下面提供代码片段

var numOfBeers = 5;
    
function getMoreBeers() {
  console.log('I have this many beers: ' + numOfBeers); // undefined
    
  var numOfBeers = 25;
  return numOfBeers;
}
    
console.log('I now have this many beers: ' + getMoreBeers());

I have this many beers: undefined
I now have this many beers: 25

console.log('I have this many beers: ' + numOfBeers);
    
var numOfBeers = 25;

VM336:2 I have this many beers: 25

console.log('I have this many beers: ' + numOfBeers);

let numOfBeers = 25;

VM333:1 Uncaught SyntaxError: Identifier 'numOfBeers' has already been declared

JavaScript ECMAScript-6 作用域控制台 .log

您正在函数中声明变量的新版本，该变量的声明将提升到调用上方，但您的赋值是在之后。等同于 .如果删除函数内部的 decaration 并仅保留赋值，则将使用在全局范围内声明的console.logfunction getMoreBeers() { var numOfBeers; console.log(...); numOfBeers = 25; }var

0赞 pilchard 10/27/2023

另请参阅： JS 提升如何在函数中工作？

答：

0赞 Mr. Polywhirl 10/27/2023 #1

因为你通过声明一个同名的内部变量来隐藏外部变量。

var numOfBeers = 5;
    
function getMoreBeers() {
  console.log('I have this many beers: ' + numOfBeers); // undefined
    
  var numOfBeers = 25; // Because you defined a local var here!
  return numOfBeers;
}
    
console.log('I now have this many beers: ' + getMoreBeers());

这里是与上面相同的代码，但变量为“hoisted”：

var numOfBeers = 5;
    
function getMoreBeers() {
  var numOfBeers; // Because you defined a local var here!

  console.log('I have this many beers: ' + numOfBeers); // undefined
    
  numOfBeers = 25; // Now we assign...
  return numOfBeers;
}
    
console.log('I now have this many beers: ' + getMoreBeers());

更改内部名称：

var numOfBeers = 5;
    
function getMoreBeers() {
  console.log('I have this many beers: ' + numOfBeers); // undefined
    
  var localNumOfBeers = 25; // Local var here!
  
  numOfBeers = localNumOfBeers; // Update the outer var
  
  return localNumOfBeers;
}
    
console.log('I now have this many beers: ' + getMoreBeers());

这将产生：

I have this many beers: 5
I now have this many beers: 25

函数中的 var 给出 undefined [duplicate]

var inside the function giving undefined [duplicate]

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