flow-tricks.js

// @flow

function getOptions() {
  return { num: 42, str: "val" };
  //     ^ Type of this return is inferred without type annotation.
}

/*::
// ^ Comments like this are part of Flow's Comment Syntax.
//   They allow you to include any additional syntax and Flow will parse it
//   like you haven't commented it out. Allowing you to write Flow code without
//   breaking JavaScript at runtime.

// Despite the syntax highlighting this is still in a comment so it won't
// execute.
var __options__ = getOptions();
//              ^ Flow will assign the inferred return type to the __options__
//                value.
               
// What we've just done is interesting because we've extracted a type from
// an arbitrary expression. Normal type annotations cannot do this (hopefully 
// in the future they will be able to).

// Now we can create a type alias with our extracted inferred type.
type Options = typeof __options__;
            // ^ typeof extracts the type from the __options__ value.
*/

// Now we can use the Options type alias as a type annotation. Our code is type
// checked:
var options /*: Options */ = {
  num: "hi", // error: string should be number
  str: 42    // error: number should be string
};

// Note that all of the above is valid JavaScript syntax. We've just been using
// comments so this is what JavaScript sees:

function getOptions() {
  return { num: 42, str: "val" };
}

var options = {
  num: "hi",
  str: 42
};