[jiaweihli]

Very early on, Typescript cleverly built up an ecosystem around community-supported type definitions for popular js libraries. This makes type-checking and integration for those libraries dead simple.

2 years later, Flow is _still_ lagging behind in this area. [1] I'm not certain, but I think this may be due to Typescript allowing for a external header-like file while Flow requires inline types.

For this reason I believe, as many other commenters have noted, Typescript has nearly always been ahead of Flow in popularity.

[1] https://github.com/facebook/flow/issues/7

[Noseshine]

I don't understand what you mean by

  this may be due to Typescript allowing for a external
  header-like file while Flow requires inline types.

Flow supports external declaration files. I use them all the time for 3rd party modules.

[jiaweihli]

You're right, this has been added recently and there's also now a community effort around 3rd party modules. [1]

Though it looks like it didn't become active until after at least 03/2016 [2].

I mention this only because TypeScript has been active in this area since 10/2012 [3], which is in-line with the original intent of my comment - that Flow is over 2 years (closer to 3.5 years) behind in building this out.

[1] https://github.com/flowtype/flow-typed

[2] https://github.com/flowtype/flow-typed/graphs/contributors

[3] https://github.com/DefinitelyTyped/DefinitelyTyped/graphs/co...