可以在后端添加以下代码：

在node.js中：

app.use((req, res, next) => {
  res.header('Access-Control-Allow-Origin', '*');
  res.header(
    'Access-Control-Allow-Headers',
    'Origin, X-Requested-With, Content-Type, Accept, Authorization'
  );
  if (req.method === 'OPTIONS') {
    res.header('Access-Control-Allow-Methods', 'PUT, POST, PATCH, DELETE, GET');
    return res.status(200).json({});
  }
  next();
});

在PHP中：

header('Access-Control-Allow-Origin: *');
header('Access-Control-Allow-Headers: Origin, X-Requested-With, Content-Type, Accept, Authorization');
header('Access-Control-Allow-Methods: GET, POST, PATCH, PUT, DELETE, OPTIONS');

通过在后端添加以上代码，可以解决Angular应用中的“No 'Access-Control-Allow-Origin' header is present on the requested resource？”问题。