It uses square-bracket notation to access the property values instead of the dot notation just in-case someone ends up using a reserved-word (e.g. `instanceof`) or an invalid JS identifier.

Note that this is only needed if backwards compatibility with ES3 engines is a concern. In ES5, this is perfectly valid code:

var foo = {
  if: 42
};
console.log(foo.if); // logs `42`

That said, even if you only care about modern engines, using square brackets is still preferable here, in case the template string contains property names that aren’t numeric literals or valid identifier names.