Question

我了解有关本地和全局变量的基础知识，但有没有人可以解释这两个代码示例之间的差异？

var elements = document.querySelectorAll('.classname');
for (var i = 0; i < elements.length; i++) {
    // do something
}

和

var elements = document.querySelectorAll('.classname');
for (i = 0; i < elements.length; i++) {                 // <-- removed "var" before "i = 0"
    // do something
}

Answer 1

使用var关键字，没有＆＃34;块＆＃34;范围，所以声明是＆＃34;功能＆＃34;范围或＆＃34;全球＆＃34;。在循环或if语句的true / false分支中声明变量的事实不会创建仅限于该代码块的变量。这与大多数编译语言完全不同。

ECMAScript 2016（a.k.a。ES6）介绍＆＃34; block＆＃34;范围包含let关键字。

因此，如果省略var关键字并在函数中创建一个新变量，那么该变量将变为全局变量，因为JS运行时并不知道您希望它的作用域。

以下是一些例子：

＆＃13;

// Outside of functions, the var keyword creates global variables
var g = "global";

function foo(){
  // Inside of functions, the var keyword creates "local" or function scoped variables
  var l = "local";
  
  // But, omitting the "var" keyword, whereever you do it, creates a global
  oops = "global";
  
  // But, creating a function scoped variable of the same name as a variable in a higher
  // scope, just hides the one from the higer scope as long as the code runs in the smaller
  // scope:
  var g = "local";

  // Other than global and function, the var keyword does not create "block" scope:
  if(true){
    var x = "surprise!";
  }
  
  // Now, we'll see what we get when we are in the local scope:
  console.log(g);     // "local", not "global" because smaller scope prevails 
  console.log(l);     // "local";
  console.log(oops);  // "global"
  console.log(x);     // "surprise!" because x has function scope, not block

}

foo();

  // Now, we'll see what is available back in the global scope:
  console.log(g);         // "global" because now we are in the global scope
  console.log(typeof l);  // It's an error to try to access directly, type is undefined
  console.log(oops);      // "global" because we omitted var and it became global
  console.log(typeof x);  // It's an error to try to access directly, type is undefined

＆＃13;

循环中的局部变量和全局变量

1 个答案: