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

我对这段代码很困惑:

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();

根据我的理解，它应该打印 0,1,2,3,4(这不是闭包的概念吗?).

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

而是打印5,5,5,5,5.

我尝试了 Rhino 和 Firefox.有人可以向我解释这种行为吗?

I tried Rhino and Firefox. Could someone explain this behavior to me?

推荐答案

通过添加额外的匿名函数修复了 Jon 的答案:

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);
  }
}

解释是 JavaScript 的作用域是函数级的，而不是块级的，创建闭包只是意味着将封闭作用域添加到封闭函数的词法环境中.

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.

循环结束后，函数级变量i 的值为5，这就是内部函数看到"的.

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

附带说明:您应该注意不必要的函数对象创建，尤其是在循环中；效率低下，如果涉及到 DOM 对象，很容易创建循环引用，从而在 Internet Explorer 中引入内存泄漏.

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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