score:6
this is not part of es6 syntax. you're looking at type definitions for static type checkers, such as flowjs or typescript. there is no es5 syntax for that.
score:7
that is typescript and it is annotating what type of value the function returns. getinitialstate returns a number.
you can see the same annotation in the arguments being passed to reduce(). this sort of annotation lets your ide/text editor do some really helpful suggestions and error checking.
Source: stackoverflow.com
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