如果已经有人问过这个问题,请告诉我。如何在每个实例上增加类变量?假设我有以下 Key 类,我想在创建实例时增加 key 变量,我尝试过:
class Key{
key = 1
constructor(){
this.key = key
Key.key++
}
print_key(){
console.log(this.key)
}
}
然后我打印 make 几个实例:
key1 = new Key()
key2 = new Key()
key3 = new Key()
key1.print_key()
key2.print_key()
key3.print_key()
期望的结果是:
1
2
3
上面的代码不起作用,我找不到具体的答案,或者某些答案似乎并不真正适合我。
最佳答案
您可以使用静态属性来记住已经创建了多少个属性,然后在初始化 key
实例属性时使用它。 static
class properties仍处于第 3 阶段,因此尚未包含在规范中,但已经相当成熟了。
class Key {
// The static property
static lastKey = 0;
// The instance property using the class fields proposal syntax
// Note I didn't initialize it with 1, that's a bit misleading.
key;
constructor() {
// Increment and assign
this.key = ++Key.lastKey;
}
print_key() {
console.log(this.key)
}
}
const key1 = new Key();
const key2 = new Key();
const key3 = new Key();
key1.print_key();
key2.print_key();
key3.print_key();
但请注意,任何地方的任何代码都可以分配给 Key.lastKey
来更改下次使用的值。
如果您想将其设为私有(private),您可以使用 private static
class field 。这些也处于第三阶段,但已经相当遥远了:
class Key {
// The static property
static #lastKey = 0;
// The instance property using the class fields proposal syntax
// Note I didn't initialize it with 1, that's a bit misleading.
key;
constructor() {
// Increment and assign
this.key = ++Key.#lastKey;
}
print_key() {
console.log(this.key)
}
}
const key1 = new Key();
const key2 = new Key();
const key3 = new Key();
key1.print_key();
key2.print_key();
key3.print_key();
Stack Snippets 没有插件来处理这个问题,所以 here's an example on the Babel REPL .
在该代码中,只有Key
代码可以访问#lastKey
。
或者只使用作用域函数:
const Key = (() => {
// Only code within this anonymous function has access to `lastKey`
let lastKey = 0;
return class Key {
// The instance property using the class fields proposal syntax
// Note I didn't initialize it with 1, that's a bit misleading.
key;
constructor() {
// Increment and assign
this.key = ++lastKey;
}
print_key() {
console.log(this.key)
}
}
})();
const key1 = new Key();
const key2 = new Key();
const key3 = new Key();
key1.print_key();
key2.print_key();
key3.print_key();
这仍然依赖于 class fields proposal (正如您在问题中所做的那样)。如果您想要一个纯 ES2015 解决方案,只需删除 key
声明即可:
const Key = (() => {
// Only code within this anonymous function has access to `lastKey`
let lastKey = 0;
return class Key {
constructor() {
// Increment and assign
this.key = ++lastKey;
}
print_key() {
console.log(this.key)
}
}
})();
const key1 = new Key();
const key2 = new Key();
const key3 = new Key();
key1.print_key();
key2.print_key();
key3.print_key();
关于Javascript - 在每个实例上增加类变量,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/59593487/