Javascript闭包？ [英] Javascript closure?

问题描述

方法＃1

function transform(ar) {
    var alStr = [];
    for(var i=0; i<ar.length; i++) {

        alStr[i] = (function(v) {
            return (function() {
                return v;
            });
        }(ar[i]));
    }

    return alStr;
}

var a = ["a", 24, { foo: "bar" }];
var b = transform(a);
a[1];
b[1]();

方法2

function transform(ar) {
    var alStr = [];
    for(var a in ar) {
        var O = function() {
            return a;
        }
        alStr.push(O);
    }
    return alStr; 
}

var a = ["a", 24, { foo: "bar" }];
var b = transform(a);
a[1];
b[1]();

上述方法用于将数组对象转换为单独的函数，目的。

The above mentioned methods are used to convert an array objects into individual functions which on execution return the specific array object. Want to know why method #1 works and method #2 doesnt.

推荐答案

在方法2中有两个问题：

In Method #2 there are two problems:

1. 您要返回键名称 a ，而不是数组值， ar [a] 。也就是说，不是 return a; 你想要 return ar [a];

1. You are returning the key name, a, rather than the array value, ar[a]. That is, rather than return a; you want return ar[a];.

该函数将始终引用最后一个循环的值，因为它引用了相同的范围对象。要创建一个新的scope对象，你需要一个闭包，一个 with block或者一个绑定的函数。

The function will always refer to the last value looped through because it references the same scope object. To create a new scope object you will need a closure, a with block, or a bound function.

使用闭包：

for(var a in ar) { 
  var O = (function(val) { 
    return function() { 
      return val; 
     }
  })(ar[a]);
  alStr.push(O); 
}

block：

for(var a in ar) { 
  with({val: ar[a]}) {
    alStr.push(function() { 
      return val; 
     });
  }
} 

使用绑定函数：

for(var a in ar) { 
  var O = function(x) { return x; };
  alStr.push(O.bind(null, arr[a]));
} 

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