Javascript isset 函数
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Javascript isset function
提问by Tamás Pap
I created an issetfunction to check if a variable is defined and not null. Here's my code:
我创建了一个isset函数来检查变量是否已定义且不为空。这是我的代码：
isset = function(a) {
if ((typeof (a) === 'undefined') || (a === null))
return false;
else
return true;
};
var a = [];
// Test 1
alert(isset(a['not']); // Alerts FALSE -> works OK
// Test 2
alert(isset(a['not']['existent'])); // Error: Cannot read property 'existent' of undefined
Any suggestion to make my function work for test 2? Thanks.
有什么建议可以让我的函数在测试 2 中工作吗？谢谢。
回答by Andrey Selitsky
You are trying to check property of an undefined object. It doesn't make any sense. You could write like this:
您正在尝试检查未定义对象的属性。这没有任何意义。你可以这样写：
alert(isset(a['not']) && isset(a['not']['existent']));
回答by Nitzan Tomer
that won't work, and you can't make it work. what happens is this: the js engine tries to evaluate a['not'] and get's "undefined", then it tries to evaluate the property 'existent' of the undefined and you get that error. all of that happens beforethe call to your function...
那是行不通的，你也做不到。发生的事情是这样的：js 引擎尝试评估 a['not'] 并获得“undefined”，然后它尝试评估 undefined 的属性“existent”，你会得到那个错误。所有这些都发生在调用您的函数之前......
what you can do is something like:
你可以做的是：
var isset = function(obj, props) {
if ((typeof (obj) === 'undefined') || (obj === null))
return false;
else if (props && props.length > 0)
return isset(obj[props.shift()], props);
else
return true;
};
then you call it like this:
然后你这样称呼它：
var a = [];
// Test 1
alert(isset(a, ['not']);
// Test 2
alert(isset(a, ['not', 'existent']));
(**this just a pseudo code, you might need to modify it a bit to actually work)
(**这只是一个伪代码，你可能需要稍微修改一下才能实际工作)
回答by Sergey Ilinsky
Test 2 will not work because "a['not']['existent']" value resolution precedes "isset" function call, and results in a runtime error.
测试 2 将不起作用，因为“a['not']['existent']”值解析在“isset”函数调用之前，并导致运行时错误。
回答by aasim
Well, You can do right this:
好吧，你可以这样做：
1) as we do in php:
1) 就像我们在 php 中所做的那样：
$vara = "abc";
$a =0;
while(isset($vara[a]){
a++;
}
2) as I do in javascript:
2）就像我在javascript中所做的那样：
vara = "abc";
while (vara[a] != null){
a++;
}
