数组迭代陷阱

Question

这段代码中出现了什么问题，我总是得到意外的输出？

var foo = [3,4,5]; 

for (var i in foo) { 
     if (i == 1) { 
    foo.unshift(6,6); 
     } 
    document.write('item: '+foo[i]+"<br>") 
    } 
output: 
item: 3 
item: 6 
item: 3 

我能得到这个一个合适的理由？感谢ü

Answer 1

我IE8得到的输出是这

item: 3 
item: 6 
item: 3 
item: 4 
item: 5 

这是正确的。如果你想在unshift使用后全面更新值另一个循环

var foo = [3,4,5]; 
    for (var i in foo) { 
     if (i == 1) { 
    foo.unshift(6,6); 
     } 
    } 
    for (var i in foo) 
    document.write('item: '+foo[i]+"<br>") 

哪位能给

item: 6 
item: 6 
item: 3 
item: 5 
item: 4 

在你的代码，当你调用document.write('item: '+foo[i]+"<br>")与i = 0你foo[0]是3 对于i=1unshiftfoo == [6,6,3,4,5]即foo[1]后是6。

Answer 2

• 从for...in摘要的mozilla

for..in should not be used to iterate over an Array where index order is important. Array indexes are just enumerable properties with integer names and are otherwise identical to general Object properties.
There is no guarantee that for...in will return the indexes in any particular order and it will return all enumerable properties, including those with non–integer names and those that are inherited.

Because the order of iteration is implementation dependent, iterating over an array may not visit elements in a consistent order. Therefore it is better to use a for loop with a numeric index (or Array.forEach or the non-standard for...of loop) when iterating over arrays where the order of access is important.

• 从ECMAScript Language Specification

If new properties are added to the object being enumerated during enumeration, the newly added properties are not guaranteed to be visited in the active enumeration. A property name must not be visited more than once in any enumeration.

让我们来讨论一下例子，如果我们正在使用一个库，它使用。

Array.prototype.maxLimit = 100000; 

该属性遍历for .. in循环。

代码的另一个版本来解释for .. in循环

var foo = [3,4,5]; 

for (var i in (alert(JSON.stringify(foo)) || foo)) { 

    if (i == 1) { 
     foo.unshift(6,6); 
    } 
    console.log('item: '+foo[i] , i , foo.length);  
} 

的alert弹出只有一次