如何在 JS 类中定义静态变量

Question

I'm building a static class to store data inside an a array and i want to declare a static variable but i don't know how to do it on javascript.

JS code:

class GCache {

    // Define cache variable
    static cache = {}; // <--- that is what i don't know how to do

    /**
     * Check if data is on cache
     * @param {*} id 
     */
    static isOnCache(id){
        return this.cache.hasOwnProperty(id);
    }

    /**
     * Add data to cache
     * @param {*} id 
     * @param {*} json 
     */
    static addToCache(id, json){
        if(this.isOnCache(id)) return;
        this.cache[id] = json;
    }

    /**
     * Obtain data from cache
     * @param {*} id 
     */
    static getFromCache(id){
        if(this.isOnCache(id)) return;
        return this.cache[id];
    }

}

Thank you <3

Answer 1

Currently* (see bottom of answer for details), non-function properties can't be added to a class itself inside the class definition. It looks ugly, but you'll have to assign the property outside of the class definition:

class GCache {
  ...
}
GCache.cache = {};

Also note that your getFromCache function probably has a bug: you probably want to return early if the id being searched for does not exist in the cache:

if(!this.isOnCache(id)) return;

 class GCache { /** * Check if data is on cache * @param {*} id */ static isOnCache(id){ return this.cache.hasOwnProperty(id); } /** * Add data to cache * @param {*} id * @param {*} json */ static addToCache(id, json){ if(this.isOnCache(id)) return; this.cache[id] = json; } /** * Obtain data from cache * @param {*} id */ static getFromCache(id){ if(!this.isOnCache(id)) return; return this.cache[id]; } } GCache.cache = {}; GCache.addToCache('myid', 'data'); console.log(GCache.getFromCache('myid'));

But, in this case, it would probably be easier to use a plain object, rather than a class. The class isn't being used to instantiate anything, after all, and with an object, you can both define cache inside the object , and reduce the syntax noise by getting rid of all the static s:

 const GCache = { cache: {}, isOnCache(id) { return this.cache.hasOwnProperty(id); }, addToCache(id, json) { if (this.isOnCache(id)) return; this.cache[id] = json; }, getFromCache(id) { if (!this.isOnCache(id)) return; return this.cache[id]; } } GCache.addToCache('myid', 'data'); console.log(GCache.getFromCache('myid'));

---

There is currently a proposal allowing you to set static non-method properties onto a class. It's currently at Stage 2, which means it's expected to eventually be implemented officially. Once it lands, the code:

class GCache {
  ...
}
GCache.cache = {};

can be replaced by:

class GCache {
  static cache = {};
  ...
}

Answer 2

如果您的意思是类的所有实例的静态变量，请在您的类声明下方声明： GCache.cache = {}

Answer 3

Babel's plugin-proposal-class-properties is probably what you're looking for. It enables support for static variables/methods inside the class declaration.