[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

[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