Flow declaration files - getting started

Question

Suppose I have this in a file named "math.js".

function double(n) {
  return n * 2;
}

exports.double = double;

If I want to declare the parameter and return type of the "double" function in a separate file named "math.js.flow", what would I put in that file?

Answer 1

You would put this in your .js.flow file:

// @flow

declare export function double(n: number): number;

Note that if your .js module contains module.exports = function foo() then you will need to do the following:

declare function foo(): void;
declare module.exports: typeof foo;

You can also generate .js.flow files with the flow gen-flow-files command. At the time of writing, it's buggy and sometimes fails, so I still write my .js.flow files manually, but if it succeeds you can look at what it output to reverse engineer more aspects of the .js.flow format.