【javascript】for-in问题总结

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for-in问题总结，具体可以参见代码，列出几种情况。

<!DOCTYPE html><html><head>    <<meta charset="utf-8">    <title>for in有问题</title></head><body>    <script type="text/javascript">    document.write("<br/>=========一般用for循环=========<br/>");    var a = []; // Create a new empty array.    a[5] = 5; // Perfectly legal JavaScript that resizes the array.    for (var i = 0; i < a.length; i++) {        // Iterate over numeric indexes from 0 to 5, as everyone expects.        document.write(a[i]+" ");    }    document.write("<br/>=========使用for-in==========<br/>");    var a = [];    a[4] = 4;    a[5] = 5;    for (var x in a) {        // Shows only the explicitly set index of "5", and ignores 0-4        document.write(a[x]+" ");    }    document.write("<br/>=========使用for-in有问题的情况===========<br/>");    // Somewhere deep in your JavaScript library...    Array.prototype.foo = 1;    // Now you have no idea what the below code will do.    var a1 = [1, 2, 3, 4, 5];    for (var x in a1) {        // Now foo is a part of EVERY array and         // will show up here as a value of 'x'.        //document.write(x+" ");        document.write("<br/>");        document.write(a1[x]+" ");    }    document.write("<br/>=========二维数组中使用for-in(将上面的注释后运行)=========");    var adj=[];    for (var i = 0; i < 10; i++) {        adj[i]=[];        adj[i].push(i);    }    adj[5].push(2);    adj[5].push(3);    adj[5].push(4);    adj[5].push(9);    for(var w in adj[5]){        document.write(w);        document.write("<br/>");         //document.write(adj[w]);    }    console.log(adj);document.write("<br/>=========使用for-in遍历对象=========");for(var x in window){            document.write(x);            document.write("<br/>");        }    </script></body></html>

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