使用Postman请求时触发TypeError: itemService未定义错误

错误信息 #

TypeError: Cannot read properties of undefined (reading 'itemService')
at /home/monts/projects/MarysPizzaTS/src/controllers/Item.ts:11:34
at Generator.next ()
at /home/monts/projects/MarysPizzaTS/src/controllers/Item.ts:8:71
at new Promise ()
at __awaiter (/home/monts/projects/MarysPizzaTS/src/controllers/Item.ts:4:12)
at getAll (/home/monts/projects/MarysPizzaTS/src/controllers/Item.ts:21:16)
at Layer.handle [as handle_request] (/home/monts/projects/MarysPizzaTS/node_modules/express/lib/router/layer.js:95:5)
at next (/home/monts/projects/MarysPizzaTS/node_modules/express/lib/router/route.js:144:13)
at Route.dispatch (/home/monts/projects/MarysPizzaTS/node_modules/express/lib/router/route.js:114:3)
at Layer.handle [as handle_request] (/home/monts/projects/MarysPizzaTS/node_modules/express/lib/router/layer.js:95:5)

原ItemController代码 #

//ItemController

import { Request, Response } from 'express';
import ItemService from '../services/Item';

class ItemController {
    private itemService: ItemService;

    constructor(itemService: ItemService) {
        this.itemService = new ItemService();
    }
    async getAll(req: Request, res: Response) {
        const items = await this.itemService.getAll();
        res.status(200).json(items);
    }
}

export default ItemController;

问题原因 #

1. this上下文丢失：Express调用getAll作为路由处理函数时，不会自动绑定类实例的this，导致this指向undefined，无法读取itemService属性。
2. 依赖注入无效：构造函数接收了itemService参数却未使用，反而重新实例化ItemService，违背了依赖注入的设计初衷。

修复方案 #

方案一：手动绑定this到实例 #

在构造函数中绑定getAll方法的this，同时修正依赖注入逻辑：

//ItemController

import { Request, Response } from 'express';
import ItemService from '../services/Item';

class ItemController {
    private itemService: ItemService;

    constructor(itemService: ItemService) {
        // 使用传入的依赖实例，而非重新创建
        this.itemService = itemService;
        // 绑定this到当前实例
        this.getAll = this.getAll.bind(this);
    }
    
    async getAll(req: Request, res: Response) {
        const items = await this.itemService.getAll();
        res.status(200).json(items);
    }
}

export default ItemController;

方案二：使用箭头函数定义方法 #

箭头函数会自动捕获外层作用域的this，无需手动绑定：

//ItemController

import { Request, Response } from 'express';
import ItemService from '../services/Item';

class ItemController {
    private itemService: ItemService;

    constructor(itemService: ItemService) {
        this.itemService = itemService;
    }
    
    // 箭头函数自动绑定this到类实例
    getAll = async (req: Request, res: Response) => {
        const items = await this.itemService.getAll();
        res.status(200).json(items);
    }
}

export default ItemController;

额外说明 #

两种方案均解决了this指向问题，同时修正了依赖注入错误，确保使用传入的ItemService实例，提升代码的可测试性和扩展性。

内容的提问来源于stack exchange，提问作者Matheus Montemurro