A default export must be at the top level of a file or module declaration.ts(1258)

This error message occurs in TypeScript when a default export is not declared at the top level of a file or module. In other words, the default export statement must be the first line of code in the file or module.

For example, if you have a file named "myModule.ts" that contains the following code:

function myFunction() {
  // code here
}

export default myFunction;

You will receive the error message because the `export default` statement is not at the top level of the module. To fix the error, move the `export default` statement to the top of the file or module:

export default function myFunction() {
  // code here
}

This ensures that the default export is declared before anything else in the file or module.