use export default and export code example

Example 1: export and export default

// Three different export styles
export foo;
export default foo;
export = foo;

// The three matching import styles
import {foo} from 'blah';
import foo from 'blah';
import * as foo from 'blah';

Example 2: export default

/*
Since export default is used to declare a fallback value for a module or 
file, you can only have one value be a default export in each module or 
file. Additionally, you cannot use export default with var, let, or const.
*/

// named function
export default function add(x, y) {
  return x + y;
}

// anonymous function
export default function(x, y) {
  return x + y;
}