Definition and Usage
定义与用法

Syntax
语法

undefined

Example
实例

In this example we will test two variables to find out if one of them is undefined:
在下面的例子中，我们将测试两个变量找出未被赋值的变量：

<script type="text/javascript">

var t1=""
var t2

if (t1==undefined) {document.write("t1 is undefined")}
if (t2==undefined) {document.write("t2 is undefined")}

</script>

The output of the code above will be:
输出结果为：

t2 is undefined

Try-It-Yourself Demos
互动演练

undefined
How to test if a variable is undefined.
如何测试一个变量是否未被赋值

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