Javascript 如何在Javascript中声明静态变量
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How to declare a static variable in Javascript
提问by indianwebdevil
In the following code, I would like to have a counter to keep track of the number of Person objects created. This code is not doing so, how would I accomplish that?
在下面的代码中，我想要一个计数器来跟踪创建的 Person 对象的数量。这段代码没有这样做，我将如何做到这一点？
function Person(){
this.name = "Peter";
this.counter = this.counter + 1;
alert(this.counter);
}
Person.prototype.counter = 0;
var p1 = new Person;
var p2 = new Person;
回答by James Allardice
function Person(){
this.name = "Peter";
Person.counter++;
alert(Person.counter);
}
Person.counter = 0;
var p1 = new Person();
var p2 = new Person();
Make the "static" variable a property of the Personfunction, rather than the prototype, and use Personinstead of thisinside the constructor.
使“静态”变量成为Person函数的属性，而不是prototype, 并在构造函数内部使用Person而不是this。
This is possible because JavaScript functions are first-class (i.e. they are objects), so can have properties of their own.
这是可能的，因为 JavaScript 函数是一流的（即它们是对象），因此可以拥有自己的属性。
Here's a working exampleof the above code.
这是上述代码的一个工作示例。
回答by Pato
You can also make your counter variable "private", declaring it as local to a closure. It's the best way to have something similar to private - static variables:
您还可以将计数器变量设为“私有”，将其声明为闭包的本地变量。这是拥有类似于私有 - 静态变量的最佳方式：
var Person = (function() {
var counter = 0;
return function() {
counter++;
this.name = "Peter";
alert(counter);
};
})();
var p1 = new Person();
var p2 = new Person();
回答by Raynos
There are no static properties. If you want you can store data on the Personfunction.
没有静态属性。如果需要，您可以在Person函数上存储数据。
function Person(){
this.name = "Peter";
Person.counter++;
alert(Person.counter);
}
回答by Alex K.
For a static you can assign a property to the function object itself;
对于静态，您可以为函数对象本身分配一个属性；
Person.counter = 0;
And within the constructor increment with;
并在构造函数中递增；
Person.counter += 1;
You can also check-if-undefined and create Person.counterwithin the constructor
您还可以检查是否未定义并Person.counter在构造函数中创建
function Person(){
if (typeof Person.counter === 'undefined')
Person.counter = 0;
else
Person.counter += 1;
...
回答by shabunc
There is no such thing like static class variables/properties in js. The simplest approach is just use the "class" function as a namespace for static variables.
js 中没有像静态类变量/属性这样的东西。最简单的方法是使用“类”函数作为静态变量的命名空间。
It means, just access in Person.count directly.
这意味着，只需直接访问 Person.count 即可。
You can use closures as well, but actually in 90% cases it will be overkill. In modern browsers you also can redefine getter/setter function to wrap usage of Person.count and other "static" variables.
你也可以使用闭包，但实际上在 90% 的情况下它会过大。在现代浏览器中，您还可以重新定义 getter/setter 函数来包装 Person.count 和其他“静态”变量的使用。
This snippet demonstrates the idea:
这个片段演示了这个想法：
function borrow(obj, borrowobj, fname) {
obj.__defineGetter__(fname, function() {
return borrowobj[fname]
})
obj.__defineSetter__(fname, function(val) {
borrowobj[fname] = val
})
}
function Person() {
borrow(this, Person, "count");
this.count++
}
Person.count = 0;
new Person();
new Person();
var p = new Person();
alert(p.count);
