找出变量是否存在 [英] Find out if a Variable exists

问题描述

我想找出一个Javascript变量是否存在.到目前为止，这是我在不同论坛上共同总结的内容:

I would like to find out if a Javascript variable exists. This is what I have so far which was cobbled together from different forums:

function valueOfVar(foo){

    var has_foo = typeof foo != 'undefined';

    if(has_foo){
        alert('1 = true');
        return true;
    }
    else {
        alert('1 = false');
        return false;
    }

}

请注意，我希望传递一个 字符串为foo.示例:valueOfVar(box_split [0] +'_ 2')

Please note, I wish to pass in a string as foo. Example: valueOfVar(box_split[0]+'_2')

现在，我不认为这可行，因为当某些变量甚至不存在时，它会返回true.实际上，它似乎一直都返回true.

Now, I don't think this works because it returns true when certain variables don't even exist. In fact, it seems to return true all the time.

一个有效的JQuery实现也很不错.

A JQuery implementation that works would also be great as I make use of this.

感谢所有帮助

推荐答案

您的意思是这样的吗?

function variableDefined (name) {
    return typeof this[name] !== 'undefined';
}

console.log(variableDefined('fred'));
// Logs "false"

var fred = 10;
console.log(variableDefined('fred'));
// Logs "true"

如果您希望能够处理局部变量，则必须做一些很奇怪的事情，像这样:

If you want to be able to handle local variables you'll have to do something quite weird like this:

function variableDefined2 (value) {
    return typeof value !== 'undefined';
}

function test() {
    var name = 'alice'
    console.log(variableDefined2(eval(name)));
    var alice = 11;
    console.log(variableDefined2(eval(name)));
}

test();
// Logs false, true

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