>Why not? They could. They syntax isn't fully specified (it does some basic syntax checking on the top level, but doesn't check anything inside brackets). Aski

>Why not? They could. They syntax isn't fully specified (it does some basic syntax checking on the top level, but doesn't check anything inside brackets). Asking browsers to support every variation of type declaration syntax that will come out is a lot (even if that turns out to be just typescript and flow). Why bloat the language and implementations? Unless you propose an unified syntax for types... in which case it will hurt innovation as we won't ever be able to make backwards breaking changes, as people will rely on all sorts of quirky behavior. >Ok, but you're ignoring the goal of this proposal: "The aim of this proposal is to enable developers to run programs written in TypeScript, Flow, and other static typing supersets of JavaScript without any need for transpilation" Right, but why is this desirable? What does this bring to the table other than some convenience in exchange for adding more bloat to the language & implementations? If you want browsers to also type-check, then you're also adding more complexity to implementations. In exchange of a small bit of convenience? If you want the type-check to be used for optimizations, while a noble goal, would require some massive reengineering of current javascript engines. It's already possible to get optimizations that come from static typing by compiling things to webassembly instead (so they can use existing pipelines, like llvm. Assemblyscript comes to mind, it tries to be similar to typescript but with mandatory types). A lot of optimizations might not even be possible to do even with type annotations, unless they're a lot more strict than typescript's. See the following examples: * https://www.typescriptlang.org/play?#code/G4QwTgBAZg9jBcEDOAXMBLAdgcwgHwkwFcAbEiAXggCIRqBuAKEfSggApYYBKCAb0YRkAUxQAxOADlSJdtyZCuAOhLCcKABb0IAeh2EYEYWDAwwjAL7MoRTAGMU6GJhHipMuf0HQ4lQjKYrIA https://www.typescriptlang.org/play?#code/G4QwTgBAZg9jBcEDOA... * https://www.typescriptlang.org/play?#code/MYGwhgzhAEBiD29oG8BQ0PQvAtgUwBcALASwDsBzALiwICdyLoAfaMgVxBGgF43OQAblQBfVKgBm7MsAIl4ZaABN4AZVyFSlABQTENBPACUKdJhITouxADps+YoxNpMr5Wo2PKAURAQ81sbCbph68HaeWhQ2IHiUxILQAPRJWETwnErQeHR08HQANNAARuwE7ngQZADkBGYYYmKS0rLyiirqDlG+-oEGiM710GERXYy8-FzCIkA https://www.typescriptlang.org/play?#code/MYGwhgzhAEBiD29oG8... * https://www.typescriptlang.org/play?#code/MYGwhgzhAEBiD29oG8BQ0PQPoXgWwFMAXACwEsA7AcwC5oIiAnSq6AH2goFcQRoBeaACISBXvCEBudJirF6+YuWoAKAJR0Gzau048+aTEegh5jeYNJkIAOhyKr1accxXb9wo9aDuvZy+hzIi5GCkDifwwAX1QY1AAzLgpgIjJ4MIATeABlB2UqFXjEOgR4NRQZDDJ46ELEG1xPfPLDAKL4BryWG1NqUkloAHpB+hJ4HgzoAkZGeEYAGmgAIy4iaCyCCAoAciJK6BiYoA https://www.typescriptlang.org/play?#code/MYGwhgzhAEBiD29oG8... JS engines already optimize number-heavy code when they can identify that variables are always going to be numbers.