Javascript 测试一个变量是否在javascript中定义？
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时间：2020-08-24 02:48:26 来源:igfitidea点击:
Test if a variable is defined in javascript?
提问by boom
How should I test if a variable is defined?
我应该如何测试变量是否已定义？
if //variable is defined
//do this
else
//do this
回答by mithunsatheesh
if (typeof variable !== 'undefined') {
// ..
}
else
{
// ..
}
find more explanation here:
在这里找到更多解释：
回答by Anurag
Use the inoperator.
使用in运算符。
'myVar' in window; // for global variables only
typeofchecks will return true for a variable if,
typeof如果，检查将为变量返回 true，
- it hasn't been defined
- it has been defined and has the value
undefined, or - it has been defined but not initialized yet.
- 它没有被定义
- 它已被定义并具有值
undefined，或 - 它已被定义但尚未初始化。
The following examples will illustrate the second and third point.
下面的例子将说明第二点和第三点。
// defined, but not initialized
var myVar;
typeof myVar; // undefined
// defined, and initialized to undefined
var myVar = undefined;
typeof myVar; // undefined
回答by Brian Antonelli
You simply check the type.
您只需检查类型。
if(typeof yourVar !== "undefined"){
alert("defined");
}
else{
alert("undefined");
}
回答by xyz
You can use something like this
你可以使用这样的东西
if (typeof varname != 'undefined')
{
//do this
}
else
{
//do this
}
