[Symbol.dispose] is a way of creating an entry whose key is the value of the expression Symbol.dispose in the same way obj[Symbol.dispose] is a way of accessing it.

The parens are just the method definition shorthand, so it’s a shorter way of writing

    [Symbol.dispose]: function()

Bracketing was introduced because Javascript was originally defined to use bare keys so

    foo: bar

Defines an entry with the key `”foo”`, rather than an entry whose key is the value for the variable `foo`. Thus to get the latter you use

    [foo]: bar