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

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.

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...

I may be biased (use and really like TS) but I think it's not even a comparison in terms of popularity

Google Trends https://www.google.com/trends/explore?date=all&q=%2Fm%2F0n50...

NPM Downloads https://npm-stat.com/charts.html?package=typescript&package=...

I don't think Typescript has ever not been bigger, but could be wrong here.

I'm having trouble remember when Flow was bigger than TS at any point?

I see 2 main reasons:

- TS has been released before Flow

- TS has the support of the Angular community

Its true and unfortunate. Typescript is the new Coffeescript. It splits the ecosystem. Flow is a progressive enhancement and improves the ecosystem. Typescript has had more push in the mindshare marketing from Microsoft.

TypeScript doesn't split the community any more than Flow does. TypeScript remains very close to ES2016/2017, syntax-wise, with the only major difference being type annotations, but you have those with flow as well. Anybody who can read JavaScript can read TypeScript.

Flow tries to integrate with the existing ecosystem as much as possible. By taking advantage of Babel, ESLint, Atom, etc.

With Flow you don't even have to opt-in to a new syntax. You can just use comments:

    function foo(val /* : boolean */) /* : string */ {}

Using really powerful inference you can also write much fewer types. If you have well types libraries [you sometimes don't need types in your code at all](https://medium.com/@thejameskyle/flow-mapping-an-object-373d...).

> With Flow you don't even have to opt-in to a new syntax. You can just use comments:

So you use a opt-in comment-based syntax instead of a opt-in non-comment based one for type-annotations.

Hardly a big difference.

> Using really powerful inference you can also write much fewer types

I'm just going to assume you didn't know typescript does this too.

Typescript is nice. You should give it a try once. You may end up surprised:)

Comments don't change it from being valid javascript whatsoever.

> By taking advantage of Babel, ESLint, Atom, etc.

Doesn't TypeScript actually predate all of those tools? So that logic doesn't seem reasonable.