javascript - 我应该如何在 JS 类中声明属性？

Question

我目前正在开发一个 API，我想创建一个类，一个从表中检索数据的 Object 类，以及一个与表关联的类。我试图在 Object 类中创建一个属性，该属性是在从与表关联的类调用静态方法 get() 时设置的。我应该如何使用这些属性？

我的 API 使用 Express 运行，我使用的是 Ubuntu 19.04，我使用 ObjectionJS 作为我的 ORM，我还使用 Knex。我有一个 loader.js 文件，它需要我的所有模型文件。

对象.js:

'use strict';

const dbTables = require('../../src/database/Models/loader');

module.exports = class Object {


    constructor () {
        console.log('Test');
    }

    //Get queries

    static get() {
        console.log(`ModelName = ${this.modelName}`);
        const req = 'dbTables.' + this.modelName + '.query()';
        return eval(req);
    }
}

UserClass.js:

'use strict';

const Object = require('./Object');

module.exports = class UserClass extends Object {

    constructor() {
        super('Employee');
        this.modelName = 'Employee';
    }
};

这是我收到的错误:

ModelName = undefined
TypeError: Cannot read property 'query' of undefined
    at eval (eval at get (/home/usersio/SafiAPI/src/Classes/Object.js:17:16), <anonymous>:1:20)
    at Function.get (/home/usersio/SafiAPI/src/Classes/Object.js:17:16)
    at app.get (/home/usersio/SafiAPI/index.js:54:28)
    at Layer.handle [as handle_request] (/home/usersio/SafiAPI/node_modules/express/lib/router/layer.js:95:5)
    at next (/home/usersio/SafiAPI/node_modules/express/lib/router/route.js:137:13)
    at Route.dispatch (/home/usersio/SafiAPI/node_modules/express/lib/router/route.js:112:3)
    at Layer.handle [as handle_request] (/home/usersio/SafiAPI/node_modules/express/lib/router/layer.js:95:5)
    at /home/usersio/SafiAPI/node_modules/express/lib/router/index.js:281:22
    at Function.process_params (/home/usersio/SafiAPI/node_modules/express/lib/router/index.js:335:12)
    at next (/home/usersio/SafiAPI/node_modules/express/lib/router/index.js:275:10)
    at methodOverride (/home/usersio/SafiAPI/node_modules/method-override/index.js:65:14)
    at Layer.handle [as handle_request] (/home/usersio/SafiAPI/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/home/usersio/SafiAPI/node_modules/express/lib/router/index.js:317:13)
    at /home/usersio/SafiAPI/node_modules/express/lib/router/index.js:284:7
    at Function.process_params (/home/usersio/SafiAPI/node_modules/express/lib/router/index.js:335:12)
    at next (/home/usersio/SafiAPI/node_modules/express/lib/router/index.js:275:10)

Answer 1

您的问题是因为您的函数 get() 是静态的，并且您试图在其上获取 this.modelName 。 但静态函数并不引用它的特定实例。

让我们看一些例子:

const dbTables = {
  Employee: {query : () => ['1', '2', '3']},
  Car: {query : () => ['A', 'B', 'C']},
}

class AbstractTable {
    //Get queries
    static get() {
        console.log(`get():`, {this:this, model: this.modelName});
        return dbTables[this.modelName] 
            && dbTables[this.modelName].query() 
            || null;
    }
    
    // Same but non static
    getNonStatic() {
        console.log(`getNonStatic():`, {this:this, model: this.modelName});
        return dbTables[this.modelName] 
            && dbTables[this.modelName].query() 
            || null;
    }
    
    // Static with args
    static getWithArg(table) {
    
        console.log(`getWithArg(table):`, {this:this, model: table.modelName, table:table});
        return dbTables[table.modelName] 
            && dbTables[table.modelName].query() 
            || null;
    }
}
class UserTable extends AbstractTable {

    constructor() {
        super('Employee');
        this.modelName = 'Employee';
    }
};

console.log("--- Test 1 ---");
console.log(UserTable.get()); // get() is static so NO instance (this is undefined)


console.log("--- Test 2 ---");
// Create an instance of UserTable: 
const userTable = new UserTable();
console.log("Function:", userTable.get); // This is not defined because get() is static  


console.log("--- Test 3 ---");
console.log("Function:", userTable.getNonStatic); 
console.log("Result:", userTable.getNonStatic()); // This is not defined because get() is static  


console.log("--- Test 4 ---");
// you can also use static and inject the UserTable
console.log("Function:", AbstractTable.getWithArg(userTable));

您还可以重新定义抽象静态函数并使用 super()

const dbTables = {
  Employee: {query : () => ['1', '2', '3']},
  Car: {query : () => ['A', 'B', 'C']},
}

class AbstractTable {
    //Get queries
    static get(modelName) {
        return dbTables[modelName] 
            && dbTables[modelName].query() 
            || null;
    }
}

class UserTable extends AbstractTable {
  static get() {
    return super.get("Employee");
  }
}

console.log(UserTable.get());