How can I rewrite path from "/api/products" to "/mock/products.json" ?

I'm using webpack for vuejs, this is my config, but not works:
```js
module.exports = {
  devServer: {
    proxy: {
      '/api': {
        target: 'http://localhost:8080',
        changeOrigin: true,
        pathRewrite(path, req) {
          return (path.replace('/api', '/mock') + '.json')
        }
      }
    }
  }
}
```
