There's also comments support in Flow directly, but it's sadly (but understandably) hideous: function somefunction(somestring/*: string*/)/*: boolean */ {

There's also comments support in Flow directly, but it's sadly (but understandably) hideous: function somefunction(somestring/*: string*/)/*: boolean */ { var ret/*: boolean */; ... return ret; } (See https://flowtype.org/blog/2015/02/20/Flow-Comments.html https://flowtype.org/blog/2015/02/20/Flow-Comments.html)