> 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/122xing-bie-yu-yun-suan-zi/1225cun-zai-yu-bu-lin.md).

# 1.2.2.5.存在與布林

* null, undefined, ""在強制型轉後會成為false
* 可利用這個特性來判斷變數是否存在

```
var a;

// goes to internet and looks for a value

if (a || a === 0) {
  console.log('Something is there.');  
}
```