Getting started with Flow

To demonstrate how to add Flow to a project, let’s set up an example npm project.

First, we’ll make an npm package for our example project called “get_started”:

$> mkdir -p get_started
$> cd get_started
$> echo '{"name": "get_started", "scripts": {"flow": "flow; test $? -eq 0 -o $? -eq 2"}}' > package.json

Next we’ll add Flow to our project:

$> touch .flowconfig
$> npm install --save-dev flow-bin

And now we can start writing some code!

index.js

// @flow


var str = 'hello world!';
console.log(str);

Note that we’ve added // @flow to the top of our file. This indicates to Flow that we want this file to be checked. If we don’t add this flag to the top of the file, Flow will assume that the file isn’t ready to be checked yet and Flow will not attempt to type check the file.

Ok, now let’s run Flow and see what it has to say about the code we just wrote:

$> npm run-script flow

> test@ flow /get_started
> flow

No errors!

Woohoo! No errors! Ok, now let’s add a trivial type error just to see what happens:

index.js

// @flow

var str: number = 'hello world!';
console.log(str);
$> npm run-script flow

> test@ flow /get_started
> flow 2> /dev/null

index.js:3
  3: let str: number = 'hello world!';
                       ^^^^^^^^^^^^^^ string. This type is incompatible with
  3: let str: number = 'hello world!';
              ^^^^^^ number


Found 1 error

Cool – Flow noticed that we’re assigning a string to a number and gave an error.

Before we fix this type error, let’s add Babel to our project so we can try running our code:

$> npm install -g babel-cli
$> npm install --save-dev babel-plugin-transform-flow-strip-types
$> echo '{"plugins": ["transform-flow-strip-types"]}' > .babelrc

Now we can try running our code in spite of the type error and see what happens:

$> babel-node index.js
hello world!

It works! As you can see Flow does not prevent you from running the code you have written even if there are type errors. It is considered best practice to never publish your project when it has type errors, but often at development time it’s useful to try running code even before it fully typechecks (usually for debugging or ad-hoc testing). This is one of the benefits of gradual typing and Flow is designed to support this and stay out of your way as much as you need it to during development.

Note that we ran our code using babel-node rather than just vanilla node to run index.js. babel-node comes with babel-cli and is just a thin wrapper around vanilla node that first intercepts and transpiles JS code before running it with node.

Check out the Running Flow Code section for more details on how we recommend compiling and publishing Flow code to npm and into production.

Next steps#

Now that we know how to set Flow up, let’s take a quick look at a few examples that are included in the Flow repo:

  1. Hello Flow!
  2. Adding type annotations
  3. Nullable types
  4. Arrays
  5. Dynamic code

Next →

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