如何检查JavaScript变量是否未定义? - Code

我尝试过的一些事情似乎并不奏效:

if(lastName != "undefined")

if(lastName != undefined)

if(undefined != lastName)

当前回答

var lastname = "Hi";

if(typeof lastname !== "undefined")
{
  alert("Hi. Variable is defined.");
}

2012-04-17 13:58:39

其他回答

var lastname = "Hi";

if(typeof lastname !== "undefined")
{
  alert("Hi. Variable is defined.");
}

2012-04-17 13:58:39

aliyun