JavaScript 類實例的靜態計數

Question

有沒有辦法保持實例的靜態計數

class Myclass {
    static s = 0;       // static property
    p = 0;              // public field declaration
    constructor() {
        console.log("new instance!")
        this.s += 1;
        this.p += 1;
        console.log(this.s, this.p);
        this.i = this.s;    // instance property
    }
}

let a = new Myclass();
console.log(a.s, a.p, a.i)

let b = new Myclass();
console.log(b.s, b.p, b.i)

輸出

new instance!
NaN 1
NaN 1 NaN
new instance!
NaN 1
NaN 1 NaN

還是在類外部更好地跟蹤實例，例如數組，例如

var instances = new Array();

class Myclass {
    constructor(name) {
        console.log("new instance!")
        this.name = name;
        this.i = instances.length;
        instances.push(this);
    }
}

let a = new Myclass('a');
console.log(instances.length, a.i)

let b = new Myclass('b');
console.log(instances.length, b.i)

console.log( instances[1].name )

與預期的輸出

new instance!
1 0
new instance!
2 1
b

Answer 1

是的，您可以使用static但不能使用this （因為它指的是特定實例）。 而是使用類名。

 class MyClass { static s = 0; // static property p = 0; // public field declaration constructor() { console.log("new instance!") MyClass.s += 1; this.p += 1; console.log(MyClass.s, this.p); this.i = MyClass.s; // instance property } } let a = new MyClass(); console.log(MyClass.s, ap, ai) let b = new MyClass(); console.log(MyClass.s, bp, bi)