[epidemian]

In JS `const` just means that you can't reassign that variable, not that the value it references can't change:

  const a = {foo: 5};
  a.foo = 42; // This is perfectly valid.
  a = 'nope'; // But this isn't. It raises a SyntaxError.

[orthoganol]

The purpose of the code wasn't to illustrate how to use const.

It was to show how they want you to always use const with complex types.

[anselmoars]

'const' is misleading, it doesn't set your variables as immutable. Fine for expression, but not for actual practicality.

[icebraining]

It does set your variables as immutable, just not your values. It makes sense if you see variables in these languages for what they are: references to values.