如何用npm Start正确创建和启动express？当我执行“npm start”时，我会得到以下错误。在WebServer@1.0.0启动脚本上失败。我在错误日志中看到了以下提示app.js的内容，但是app.js有什么不正确的地方？谢谢。

20 error code ELIFECYCLE
21 error errno 126
22 error webserver@1.0.0 start: `./src/app.js`
22 error Exit status 126
23 error Failed at the webserver@1.0.0 start script.

我有以下package.json文件。

{
    "name": "webserver",
    "preferGlobal": true,
    "version": "1.0.0",
    "author": "Milton Centeno <centem@gmail.com>",
    "description": "a simple express web server",
    "license": "MIT",
    "main" : "./src/app.js",
    "engines": {
        "node": ">=0.10"
    },
    "scripts": {
        "test": "echo \"Error: no test specified\" && exit 1"
    },
    "dependencies": {
        "express": ">=4.15.3"
    }
}

下面是tree命令的输出，它显示了app.js文件相对于package.json的位置。

.
├── node_modules
├── package-lock.json
├── package.json
└── src
    └── app.js

这就是我的app.js所用的

// Load the Express package as a module
const express = require("express");

// Access the exported service
const app = express();

// Return a string for requests to the root URL ("/")
app.get("/", (request, response) => {
  response.send("Hello from Express!");
});

// Start listening to incoming requests
// If process.env.PORT is not defined, port number 3000 is used
const listener = app.listen(process.env.PORT || 3000, () => {
  console.log(`Your app is listening on port ${listener.address().port}`);
});