In most languages, doing what this article describes is quite straightforward: you would just define a new type (/ struct / class) called ‘Hash’, which functions can take or return. The language automatically treats this as a completely new type. This is called ‘nominal typing’: type equality is based on the name of the type.
The complication with TypeScript is that it doesn’t have nominal typing. Instead, it has ‘structural typing’: type equality is based on what the type contains. So you could define a new type ‘Hash’ as a string, but ‘Hash’ would just be a synonym — it’s still considered interchangeable with strings. This technique of ‘branded types’ is simply a way to simulate nominal typing in a structural context.
The complication with TypeScript is that it doesn’t have nominal typing. Instead, it has ‘structural typing’: type equality is based on what the type contains. So you could define a new type ‘Hash’ as a string, but ‘Hash’ would just be a synonym — it’s still considered interchangeable with strings. This technique of ‘branded types’ is simply a way to simulate nominal typing in a structural context.