JavaScript不支持带有局部变量的闭包吗？ [英] Doesn't JavaScript support closures with local variables?

问题描述

我对此代码非常困惑：

  var closures = []; 
 function create（）{
 for（var i = 0; i <5; i ++）{
 closureures [i] = function（）{
 alert + i）; 
}; 
} 
} 
 
 function run（）{
 for（var i = 0; i <5; i ++）{
 closures [i] （）; 
} 
} 
 
 create（）; 
 run（）; 
  

从我的理解它应该打印0,1,2,3,4（不是这个

I尝试Rhino和Firefox。

有人可以向我解释这个行为吗？
提前Thx。

解决方案

通过添加一个额外的匿名函数修复Jon的回答：

  function create（）{
 for（var i = 0; i <5; i ++）{
 closureures [i] （function（tmp）{
 return function（）{
 alert（i =+ tmp）; 
}; 
} 
} 
} 
  

解释是JavaScript的范围是函数级，不是块级的，并且创建闭包只是意味着封闭范围被添加到封闭函数的词法环境。

循环终止后，函数级变量 i 具有 5 的值，这是内部函数看到的。

---

注意：你应该注意不必要的函数对象创建，特别是在循环中;它的效率低，如果涉及DOM对象，很容易创建循环引用，因此在Internet Explorer中引入内存泄漏。

I am very puzzled about this code:

var closures = [];
function create() {
  for (var i = 0; i < 5; i++) {
    closures[i] = function() {
      alert("i = " + i);
    };
  }
}

function run() {
  for (var i = 0; i < 5; i++) {
    closures[i]();
  }
}

create();
run();

From my understanding it should print 0,1,2,3,4 (isn't this the concept of closures?).

Instead it prints 5,5,5,5,5.

I tried Rhino and Firefox.

Could someone explain this behavior to me? Thx in advance.

解决方案

Fixed Jon's answer by adding an additional anonymous function:

function create() {
  for (var i = 0; i < 5; i++) {
    closures[i] = (function(tmp) {
        return function() {
          alert("i = " + tmp);
        };
    })(i);
  }
}

The explanation is that JavaScript's scopes are function-level, not block-level, and creating a closure just means that the enclosing scope gets added to the lexical environment of the enclosed function.

After the loop terminates, the function-level variable i has the value 5, and that's what the inner function 'sees'.

---

As a side note: you should beware of unnecessary function object creation, espacially in loops; it's inefficient, and if DOM objects are involved, it's easy to create circular references and therefore introduce memory leaks in Internet Explorer.

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