[id].js

// this code belongs to some dynamic route page, like /products/:id

export async function getStaticPaths() {
  // some back end code here
  return {
    paths: [
      { params: { ... } } 
    ],
    fallback: true or false 
  };
}

// Here is an example of the shape of the data that must be returned by this function:
return {
  paths: [
    { params: { id: '1' } },
    { params: { id: '2' } }
  ],
  fallback: ...
}