TIL: April 2020

• TypeScript custom type guards

function isFish(pet: Fish | Bird): pet is Fish {
  return (pet as Fish).swim !== undefined;
}

• TypeScript mapped types, keyof and lookup types

type Readonly<T> = {
  readonly [P in keyof T]: P[T];
}

• JS: typeof null => object (why: https://2ality.com/2013/10/typeof-null.html)

• Deploying Rails with webpack to Heroku (from Rails 5.1 loves Javascript)

heroku create
heroku buildpacks:add --index 1 heroku/nodejs
heroku buildpacks:add --index 2 heroku/ruby
git push heroku master

• Chess: En passant

1. by sobstel • April 2020