调用回调函数而无需在 JavaScript 中额外声明箭头函数 [重复]

express上有一个路由器：

import Router from "express";
import ModbusController from "../Controllers/ModbusController.js";

const mb = new ModbusController();

const modbusRouter = new Router();

modbusRouter.get('/write/:id/:action', (req, res) => mb.writeSingleCoil(req, res));
modbusRouter.get('/read/:id', (req, res) => mb.readSingleCoil(req, res));
modbusRouter.get('/read', (req, res) => mb.readAllCoils(req, res));

export default modbusRouter;

有一个 ModbusController 类：

import modbus from "jsmodbus";
import net from "net";
import dotenv from "dotenv";

export default class ModbusController {
// много неважного кода
  readSingleCoil(req, res) {
    const {id} = req.params
    this.client.readCoils(id,1)
      .then( resp => {
        const coilStatus = resp.response._body._valuesAsArray[0];
        res.status(200).json({ch: id, status: coilStatus})
      })
      .catch(e => this.errorOccurred(e))
  }

这就是一切的工作原理，但我几乎 100% 确定从控制器调用函数的行可以这样写：

modbusRouter.get('/read/:id', mb.readSingleCoil);

为什么我会收到错误：'(node:11488) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'readSingleCoil' of undefined'？