Using Flow, Babel and gulp to type-check ES6 code (with sourcemaps)_
Apr 27, 2015

TL;DR - this post shows a method to use Flow, Babel and Gulp to transpile and type-check ES6 code, with support for sourcemaps in the resulting messages.

Babel, flow

A lot of the annoyances of developing with Javascript are starting to go away. In particular, ES6 has removed a lot of the fluff from JS and made the language a lot more elegant and fun to code in.

The final missing piece of the Javascript puzzle are types. Javascript’s dynamic typing is both a strength and a weakness - its extremely expressive and powerful, allowing for very a lot of interesting abstractions. At the same time, things are so flexible that its very easy to shoot yourself in the foot without realising it.

Flow, babel

The boffins at Facebook have come to the same conclusion and have been working on Flow, which is a static typechecker for Javascript. I find it very nice for a number of reasons, including these:

Putting it all together

My goal was to integrate flow typechecking into my build process (in my case this is gulp, but I am sure that the techniques here can be ported to grunt or whatever else you might be using). Here is my final gulpfile.js showing the flow and flow:watch tasks. Note that this assumes that your ES6 source code is in a directory called src.

var gulp = require("gulp"),
  notify = require("gulp-notify"),
  babel = require("gulp-babel"),
  flow = require("gulp-flowtype"),
  sourcemaps = require("gulp-sourcemaps"),
  sourcemapReporter = require("jshint-sourcemap-reporter");

var clientSrcDir = "src",
  flowDest = "tmp_build_flow";

gulp.task("flow:babel", function (cb) {
    .src(clientSrcDir + "/**/*.js")
    .pipe(babel({ blacklist: ["flow"] }))
    .on("error", notify.onError("<%= error.message %>"))
    .on("end", cb);

gulp.task("flow", ["flow:babel"], function () {
    .src(flowDest + "/**/*.js")
        all: false,
        weak: false,
        killFlow: false,
        beep: true,
        abort: false,
        reporter: {
          reporter: function (errors) {
            return sourcemapReporter.reporter(errors, {
              sourceRoot: "/" + clientSrcDir + "/",

gulp.task("flow:watch", function () { + "/**/*.js", ["client:flow"]);

gulp-flowtype is a gulp plugin that runs flow as part of a gulp pipeline. It can be installed from npm using:

npm install gulp-flowtype --save

I have also built a simple reporter that supports sourcemaps. Furthermore if you happen to be using IntellJ or WebStorm and run the task through the builtin Gulp runner you should be able to click on the error messages from gulp and be taken directly to the position in the original, un-transpiled, file. Install the reporter with:

npm install jshint-sourcemap-reporter --save

Finally it is necessary to add the ES6 transpilation directory to Flow’s source, and to ignore the ES6 source directory by editing the .flowconfig (created when you run flow init). Therefore this should read:





Note that if you fancy you can extends this pipeline further to do browerifying, concatentation, uglfying, etc, but personally I leave these tasks like this and have separate tasks for other purposes.