Use const keyword for immutable variables

README.md

@@ -110,9 +110,9 @@ Monitoring the advertising packets, you can find your devices and know the lates
 
 ```JavaScript
 // Load the node-switchbot and get a `Switchbot` constructor object
-let Switchbot = require('node-switchbot');
+const Switchbot = require('node-switchbot');
 // Create an `Switchbot` object
-let switchbot = new Switchbot();
+const switchbot = new Switchbot();
 
 (async () => {
   // Start to monitor advertisement packets
@@ -137,9 +137,9 @@ The [`startScan()`](#startscan-method) and [`wait()`](#Switchbot-wait-method) me
 
 ```javascript
 // Load the node-switchbot and get a `Switchbot` constructor object
-let Switchbot = require("node-switchbot");
+const Switchbot = require("node-switchbot");
 // Create an `Switchbot` object
-let switchbot = new Switchbot();
+const switchbot = new Switchbot();
 
 // Start to monitor advertisement packets
 switchbot
@@ -213,18 +213,18 @@ This sample discovers a Bot (WoHand), then put the Bot's arm down, finally put i
 
 ```javascript
 // Load the node-switchbot and get a `Switchbot` constructor object
-let Switchbot = require("node-switchbot");
+const Switchbot = require("node-switchbot");
 // Create an `Switchbot` object
-let switchbot = new Switchbot();
+const switchbot = new Switchbot();
 
 (async () => {
   // Find a Bot (WoHand)
-  let bot_list = await switchbot.discover({ model: "H", quick: true });
+  const bot_list = await switchbot.discover({ model: "H", quick: true });
   if (bot_list.length === 0) {
     throw new Error("No device was found.");
   }
   // The `SwitchbotDeviceWoHand` object representing the found Bot.
-  let device = bot_list[0];
+  const device = bot_list[0];
   // Put the Bot's arm down (stretch the arm)
   await device.down();
   // Wait for 5 seconds
@@ -246,13 +246,13 @@ In this code, you can get a [`SwitchbotDeviceWoHand`](#SwitchbotDeviceWoHand-obj
 In order to use the node-switchbot, you have to load the node-switchbot module as follows:
 
 ```JavaScript
-let Switchbot = require('node-switchbot');
+const Switchbot = require('node-switchbot');
 ```
 
 You can get an `Switchbot` constructor from the code above. Then you have to create an `Switchbot` object from the `Switchbot` constructor as follows:
 
 ```javascript
-let switchbot = new Switchbot();
+const switchbot = new Switchbot();
 ```
 
 The `Switchbot` constructor takes an argument optionally. It must be a hash object containing the properties as follows:
@@ -263,15 +263,15 @@ The `Switchbot` constructor takes an argument optionally. It must be a hash obje
 
 The node-switchbot module uses the [`@abandonware/noble`](https://github.com/abandonware/noble) module in order to interact with BLE devices. If you want to interact other BLE devices using the `@abandonware/noble` module, you can create an `Noble` object by yourself, then pass it to this module. If you don't specify a `Noble` object to the `noble` property, this module automatically create a `Noble` object internally.
 
-The sample code below shows how to pass a `Nobel` object to the `Switchbot` constructor.
+The sample code below shows how to pass a `Noble` object to the `Switchbot` constructor.
 
 ```JavaScript
 // Create a Noble object
-let noble = require('@abandonware/noble');
+const noble = require('@abandonware/noble');
 
 // Create a Switchbot object
-let Switchbot = require('node-switchbot');
-let switchbot = new Switchbot({'noble': noble});
+const Switchbot = require('node-switchbot');
+const switchbot = new Switchbot({'noble': noble});
 ```
 
 In the code snippet above, the variable `switchbot` is an `Switchbot` object. The `Switchbot` object has a lot of methods as described in sections below.
@@ -565,7 +565,7 @@ The code below calls the [`press()`](#SwitchbotDeviceWoHand-press-method) method
 switchbot
   .discover({ model: "H", quick: true })
   .then((device_list) => {
-    let device = device_list[0];
+    const device = device_list[0];
     if (!device) {
       console.log("No device was found.");
       process.exit();

lib/parameter-checker.js

@@ -37,7 +37,7 @@ class ParameterChecker {
    *   an `Error` object will be set to `this._error`.
    *
    * [Usage]
-   * let valid = parameterChecker.check(params, {
+   * const valid = parameterChecker.check(params, {
    *   level: {
    *     required: false,
    *     type: 'integer',
@@ -50,7 +50,7 @@ class ParameterChecker {
    *   }
    * });
    * if(!valid) {
-   *   let e = parameterChecker.error.message;
+   *   const e = parameterChecker.error.message;
    *   throw new Error(message);
    * }
    * ---------------------------------------------------------------- */
@@ -79,11 +79,11 @@ class ParameterChecker {
     }
 
     let result = true;
-    let name_list = Object.keys(rules);
+    const name_list = Object.keys(rules);
 
     for (let i = 0; i < name_list.length; i++) {
-      let name = name_list[i];
-      let v = obj[name];
+      const name = name_list[i];
+      const v = obj[name];
       let rule = rules[name];
 
       if (!rule) {
@@ -458,7 +458,7 @@ class ParameterChecker {
       }
     }
     if (typeof rule.minBytes === "number") {
-      let blen = Buffer.from(value, "utf8").length;
+      const blen = Buffer.from(value, "utf8").length;
       if (blen < rule.minBytes) {
         this._error = {
           code: "LENGTH_UNDERFLOW",
@@ -475,7 +475,7 @@ class ParameterChecker {
       }
     }
     if (typeof rule.maxBytes === "number") {
-      let blen = Buffer.from(value, "utf8").length;
+      const blen = Buffer.from(value, "utf8").length;
       if (blen > rule.maxBytes) {
         this._error = {
           code: "LENGTH_OVERFLOW",