[octref]

Did you also consider Flow[0] as an alternative? I'm now using ES6 with Babel to build some small side-projects and it has been a great experience. But as the codebase grows, I'd appreciate to add some Flow type annotations. Typescript looks great but it's still not JS. I wonder what will happen to all those compile-to-JS languages once ES6 becomes supported everywhere.

[0]: http://flowtype.org/

[chimeracoder]

> I'd appreciate to add some Flow type annotations. Typescript looks great but it's still not JS.

The Flow type annotations are almost identical to Typescript. There's one edge case around one of them requiring a space before/after the colon and the other not, but I can't remember what it is. I just tried running one of the Flow examples through the Typescript playground and it worked fine[0].

The biggest differences between Flow and Typescript are how you run the build system (`tsc` versus the Flow server) and the file extension you put on the file.

[0] http://www.typescriptlang.org/Playground#src=%0A%2F%2F%20htt...

[samwgoldman]

While the type annotation syntax between Flow and Typescript are mostly identical (this is intentional), there are a bunch of differences between them:

* TypeScript allows unsound casting, Flow doesn't. These casts are very practical, as you might know more than the analyzer. Flow takes a stricter position here, which is a theme.

* Function are bivariant w.r.t. their parameters (which is unsound) in TypeScript, but contravariant in Flow. Again, this is an intentional, practical choice, but Flow emphasizes soundness.

* TypeScript asks users to add annotations in scenarios where Flow will infer types. TypeScript will infer any (there is a "no implicit any" option).

* Classes in Flow are nominal (interfaces are structural). Classes are structural in TypeScript.

* Flow adds no additional runtime syntax to JavaScript; TypeScript does (enums, for example). Flow does support some ES2016 features (async/await, object spread), but generally holds off on experimental features (stage < 2).

* Flow has a couple interesting features absent in TypeScript, like disjoint unions. I suspect/hope both systems will converge on useful features like this.

* TypeScript treats null as a bottom type, but Flow uses explicit "maybe types."

[jiaweihli]

TypeScript supports disjoint unions: http://blogs.msdn.com/b/typescript/archive/2015/09/16/announ...

The two real pain points (among those you mentioned) of TypeScript are bivariant function parameters and structural classes, since they lead to potentially unsound code that compiles.

[samwgoldman]

The link you provided doesn't seem to show disjoint union support. I'm talking about this: http://flowtype.org/blog/2015/07/03/Disjoint-Unions.html

[octref]

Thanks a lot for your input. I think I'll stay with ES6 and flow.