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JavaScript undefined Property

<JavaScript的全局函数

测试一个变量是不确定的:

var x;

if (x === undefined) {
    txt = "x is undefined";
} else {
    txt = "x is defined";
}

TXT的结果将是:

x is undefined
试一试»

更多"Try it Yourself"下面的例子。


定义和用法

未定义属性指示变量尚未赋值。


浏览器支持

属性
undefined

技术细节

JavaScript的版本: 1.3

例子

更多示例

测试如果变量是不确定的:

var t1 = "myVar";               // defined
var t2;                         // undefined

if (t1 === undefined) {
    txt1 = "t1 is undefined";
} else {
    txt1 = "t1 is defined";
}

if (t2 === undefined) {
    txt2 = "t2 is undefined";
} else {
    txt2 = "t2 is defined";
}

txt = txt1 + "<br>" + txt2;

TXT的结果将是:

t1 is defined
t2 is undefined
试一试»

<JavaScript的全局函数