> For the complete documentation index, see [llms.txt](https://jen-hsuan-hsieh.gitbook.io/javascript-node-js/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jen-hsuan-hsieh.gitbook.io/javascript-node-js/chapter1/12javascript-quan-gong-lve-ff1a-ke-fu-js-de-qi-guai-bu-fen/125han-shu-jian-gou-zi-3001-300c-new-300d-yu-javascript-de-li-shi/1256wei-xian-xiaoding-ning-ff1a-zhen-lie-yu-for-in.md).

# 1.2.5.6.危險小叮嚀：陣列與 for in

* 陣列是物件, 而成員是被新增的屬性

```
var arr = ['John', 'Jane', 'Jim']


for (var prop in arr) {
    console.log(prop + ':' + arr[prop])
}


Array.prototype.myCustomFeature = 'cool!'
```