如何使用koa.js提供静态文件(如果存在)和带有默认值的文件(如果不存在

npm install koa koa-static

const Koa = require('koa');
const static = require('koa-static');

const app = new Koa();

const staticPath = './public';
const staticOptions = {};

app.use(static(staticPath, staticOptions));

const fs = require('fs');
const path = require('path');

const defaultFilePath = './default.html';

app.use(async (ctx, next) => {
  const filePath = path.join(staticPath, ctx.path);
  
  if (!fs.existsSync(filePath)) {
    ctx.type = 'text/html';
    ctx.body = fs.readFileSync(defaultFilePath, 'utf8');
  } else {
    await next();
  }
});

const port = 3000;

app.listen(port, () => {
  console.log(`Server is running on port ${port}`);
});