javascript/jQuery 中是否有类似 php 的isset 之类的东西？

is there something like isset of php in javascript/jQuery?

提问by gautamlakum

Is there something in javascript/jQuery to check whether variable is set/available or not? In php, we use isset($variable)to check something like this.

javascript/jQuery 中有什么东西可以检查变量是否设置/可用？在 php 中，我们isset($variable)用来检查这样的事情。

thanks.

谢谢。

回答by Emil Vikstr?m

Try this expression:

试试这个表达式：

typeof(variable) != "undefined" && variable !== null

This will be true if the variable is defined and not null, which is the equivalent of how PHP's isset works.

如果变量已定义且不为空，则为 true，这与 PHP 的 isset 的工作方式等效。

You can use it like this:

你可以这样使用它：

if(typeof(variable) != "undefined" && variable !== null) {
    bla();
}

回答by Oleg

JavaScript isset() on PHP JS

PHP JS 上的 JavaScriptisset()

function isset () {
    // discuss at: http://phpjs.org/functions/isset
    // +   original by: Kevin van     Zonneveld (http://kevin.vanzonneveld.net)
    // +   improved by: FremyCompany
    // +   improved by: Onno Marsman
    // +   improved by: Rafa? Kukawski
    // *     example 1: isset( undefined, true);
    // *     returns 1: false
    // *     example 2: isset( 'Kevin van Zonneveld' );
    // *     returns 2: true
    var a = arguments,
        l = a.length,
        i = 0,
        undef;

    if (l === 0) {
        throw new Error('Empty isset');
    }

    while (i !== l) {
        if (a[i] === undef || a[i] === null) {
            return false;
        }
        i++;
    }
    return true;
}

回答by zzzzBov

If you want to check if a property exists: hasOwnPropertyis the way to go

如果你想检查一个属性是否存在：hasOwnProperty是要走的路

And since most objects are properties of some other object (eventually leading to the windowobject) this can work well for checking if values have been declared.

并且由于大多数对象是某个其他对象的属性（最终导致该window对象），因此可以很好地检查值是否已声明。

回答by DanneManne

typeof will serve the purpose I think

typeof 将达到我认为的目的

if(typeof foo != "undefined"){}

回答by stuyam

Some parts of each of these answers work. I compiled them all down into a function "isset" just like the question was asking and works like it does in PHP.

每个答案的某些部分都有效。我将它们全部编译成一个函数“isset”，就像问题所问的那样，并且像在 PHP 中一样工作。

// isset helper function 
var isset = function(variable){
    return typeof(variable) !== "undefined" && variable !== null && variable !== '';
}

// isset helper function 
var isset = function(variable){
    return typeof(variable) !== "undefined" && variable !== null && variable !== '';
}

Here is a usage example of how to use it:

以下是如何使用它的用法示例：

var example = 'this is an example';
if(isset(example)){
    console.log('the example variable has a value set');
}

It depends on the situation you need it for but let me break down what each part does:

这取决于您需要它的情况，但让我分解每个部分的作用：

1. typeof(variable) !== "undefined"checks if the variable is defined at all
2. variable !== nullchecks if the variable is null (some people explicitly set null and don't think if it is set to null that that is correct, in that case, remove this part)
3. variable !== ''checks if the variable is set to an empty string, you can remove this if an empty string counts as set for your use case

1. typeof(variable) !== "undefined"检查变量是否已定义
2. variable !== null检查变量是否为空（有些人明确设置为空，不认为它是否设置为空是正确的，在这种情况下，删除这部分）
3. variable !== ''检查变量是否设置为空字符串，如果空字符串计为您的用例设置，您可以删除它

Hope this helps someone :)

希望这对某人有所帮助:)

回答by Ioannis Karadimas

Not naturally, no... However, a googling of the thing gave this: http://phpjs.org/functions/isset:454

不自然，不......但是，谷歌搜索给出了这个：http: //phpjs.org/functions/isset: 454

回答by Sandeepan Nath

http://phpjs.org/functions/isset:454

http://phpjs.org/functions/isset:454

phpjs project is a trusted source. Lots of js equivalent php functions available there. I have been using since a long time and found no issues so far.

phpjs 项目是一个值得信赖的来源。那里有很多 js 等效的 php 函数。我已经使用了很长时间，到目前为止没有发现任何问题。

回答by Dieter Gribnitz

The problem is that passing an undefined variable to a function causes an error.

问题是将未定义的变量传递给函数会导致错误。

This means you have to run typeof before passing it as an argument.

这意味着您必须在将其作为参数传递之前运行 typeof。

The cleanest way I found to do this is like so:

我发现这样做的最干净的方法是这样的：

function isset(v){
    if(v === 'undefined'){
        return false;
    }
    return true;
}

Usage:

用法：

if(isset(typeof(varname))){
  alert('is set');
} else {
  alert('not set');
}

Now the code is much more compact and readable.

现在代码更加紧凑和可读。

This will still give an error if you try to call a variable from a non instantiated variable like:

如果您尝试从非实例化变量中调用变量，这仍然会产生错误，例如：

isset(typeof(undefVar.subkey))

thus before trying to run this you need to make sure the object is defined:

因此，在尝试运行之前，您需要确保已定义对象：

undefVar = isset(typeof(undefVar))?undefVar:{};

回答by Mr Bill

Here :)

这里 ：）

function isSet(iVal){
 return (iVal!=="" && iVal!=null && iVal!==undefined && typeof(iVal) != "undefined") ? 1 : 0;
} // Returns 1 if set, 0 false

回答by user2619604

in addition to @emil-vikstr?m's answer, checking for variable!=nullwould be true for variable!==nullas well as for variable!==undefined(or typeof(variable)!="undefined").

除了@emil-vikstr?m的答案之外，检查 for以及 for (or )variable!=null都是真的。variable!==nullvariable!==undefinedtypeof(variable)!="undefined"