flow-runtime-experiment

flow-runtime allows us to add runtime type-checks with a lean syntax.

For example, this:

const add = (x, y) => {
  if (typeof x != 'number') {
    throw new Error('x must be a number');
  }
  if (typeof y != 'number') {
    throw new Error('y must be a number');
  }
  return x + y;
}

Can be written like this:

const add = (x : number, y : number) : number => {
  return x + y;
}

Demo

yarn install --pure-lockfile 

First add type definitions to your code. For example:

cat ./src/index.js

Build the library using Babel:

yarn build

This generates standard ES code that you can run in Node:

cat ./lib/index.js

node
> require('./lib').add(1, 2)
3

And flow-runtime gives us type checks:

node
> require('./lib').add(1, 'world')
RuntimeTypeError: y must be a number

Expected: number

Actual Value: "world"

Actual Type: string