Flow or Type - how to React to that?

Flow or Type - how
to React to that?
Domagoj Cerjan
Kresimir Antolic

Nothing revolutionary.
Hints and cases where *THIS* can help you!
Enough for you to start believing in the TYPE.

● Familiar with these?
● What is the second argument of that function?
● I removed a function from a module, what code used it and where will it break?

OUR LORD AND SAVIOUR…*
* Not really but it helps A LOT.

● A safeguard preventing us from assigning apples to oranges
● A tool which helps to avoid ‘undefined is not a function’ or ‘cannot read property x of
undefined’ errors
● Gives a sense of hope and security in huge codebases
● Makes refactoring a little less heart attack prone
● Makes code more clear and explicit
A Type System!

… Not exactly.
● Around the React ecosystem… Yeah
● Not tied directly but it makes our life easier
● Defining input for your components and describing your data

API DATA
TRANSFORM
DATA
API DATA
TRANSFORM
DATA
COMBiNE
TRANSFORM
UI
REACT
COMPONENT
LOCAL
DATA
INTERACTION!
MODIFY
FUNCTION
CALL
DATA

Flow
● Type-checking preprocessor bolted
onto JS
● More oriented towards FP
● Requires a type annotation removal
step before JS code can be consumed
(usually via @babel/preset-flow)
Typescript
● Actual language with its own
compiler
● More oriented towards OOP
● Has enums (it is a different
language)
● Requires a compilation step to
produce runnable JS code (via tsc)
Type System{s}?

Flow
● Prioritizes soundness - it rejects all
invalid code and some valid
● Type-related bugs do not slip through
● Objects, Interfaces and Functions are
structurally typed
● Classes are nominally typed (ie two
differently named classes having exact
same shape are different)
Typescript
● Prioritizes completeness - accepts
all valid code and some invalid
● Type-related bugs might slip
through
● Objects and Interfaces are
structurally typed
● Classes, Functions and somewhat
Enums are nominally typed
Type System{s}? - Continued

Making sure an apple is an apple
Types

let call: number = 'maybe';
// Type '"maybe"' is not assignable to type 'number'
call = 2;
class Apple { private name: string; }
class Orange { private name: string; }
let thing: Apple;
thing = new Apple();
thing = new Orange();
// Type 'Orange' is not assignable to type 'Apple'.
// Types have separate declarations of a private property 'name'.
Typescript

let call: number = 'maybe';
// Cannot assign 'maybe' to call because string [1] is incompatible with number [2].
call = 2;
class Apple { _name: string; }
class Orange { _name: string; }
let thing: Apple;
thing = new Apple();
thing = new Orange();
// Cannot assign new Orange() to thing because Orange [1] is incompatible with Apple [2].
Flow

Describing contents in the box
Interfaces/Types

type PokemonType = 'water' | 'fire' | 'electric';
interface IPokemon {
name: string;
type: PokemonType[];
}
interface IProps {
list: IPokemon[];
}
interface IState {
isExpanded: boolean;
}
export class PokemonList extends React.PureComponent<IProps, IState> {
state = { isExpanded: false };
render() {
const { list } = this.props;
return list ? (<div>{list.map((p) => <div>{p.name}</div>)}</div>) : null;
}
}
Typescript

type PokemonType = 'water' | 'fire' | 'electric';
interface IPokemon {
name: string;
type: PokemonType[];
}
interface IProps {
list: IPokemon[];
}
interface IState {
isExpanded: boolean;
}
export class PokemonList extends PureComponent<IProps, IState> {
state = { isExpanded: false };
render() {
const { list } = this.props;
return list ? (<div>{list.map((p) => <div>{p.name}</div>)}</div>) : null;
}
}
Flow

Where type systems shine
Inference

class Apple { private name: string; }
class Orange { private name: string; }
const fruitBox = [new Orange(), new Orange(), new Apple()]
const makeOrangeJuice = (oranges: Orange[]): string =>
`Orange juice (${oranges.length})dl`;
makeOrangeJuice(fruitBox);
// Type 'Apple' is not assignable to type 'Orange'.
// Types have separate declarations of a private property 'name'.
Typescript

class Apple { _name: string; }
class Orange { _name: string; }
const fruitBox = [new Orange(), new Orange(), new Apple()]
const makeOrangeJuice = (oranges: Orange[]): string =>
`Orange juice (${oranges.length})dl`;
makeOrangeJuice(fruitBox);
// Cannot call makeOrangeJuice with fruitBox bound to oranges because Apple [1] is incompatible with Orange [2] in
array element.
Flow

:any is the enemy, believe in type inference
● Using :any type turns type inference off, any is bad, don’t be like any
● We don’t want to type types manually everywhere
● Humans make mistakes often, some developers do also
● Type inference systems usually don’t, but when they do, they do it consistently
● Do not assume types, let type inference do that for you!
● Relying on type inference mechanisms allows us to think less and code more

Organizing your boxes
Structuring your app

export class Pokemon {
static readonly type: string = 'electric';
public height: number = 0.5;
protected pokemonIdx: number = 0;
private name: string = 'pikachu';
constructor() {
console.log(this.name);
}
}
Typescript

export class Pokemon {
static +type: string = 'electric';
height: number = 0.5;
pokemonIdx: number = 0;
_name: string = 'pikachu';
constructor() {
console.log(this._name);
}
}
Flow

Working with the codebase
Stick, stones, wood, lava..

Flow
● IDE integration is lacking
compared to Typescript
● Slow, though improving a lot with
every new release
● Outstanding command line tools
● ESLint with flow-type plugin
Typescript
● Great IDE integration (Idea, VSCode...)
● Fast
● Excellent code-completion facilities
● Ok-ish command line tools
● Parallelised type checking
● TSLint (though lacking in comparison
with ESLint)
Working with the codebase

Working well with others?
● Both flow and typescript offer facilities to define library typings
○ Flow via *.flow.js files
○ Typescript via *.d.ts files
● Most commonly used libraries (lodash, react, redux…) have well-tested
and defined typings which provide both code completions and type-safety
● You can always extend, override or write yours

It's leviosa not leviosa
Advanced usages

Advanced usages
● Code generation!
○ Gives type safety on multiple levels
● Generic types
● Nominal vs Structural typing
● Type unions and intersections

GREAT - LET’S MOVE TO TYPES!

When to migrate?
When to migrate to either Typescript or Flow
● Big crucial project
● High people turnover rate
● Aiming to increase robustness of the project codebase
● OOP oriented - Typescript
● FP oriented - Flow
When not to think about it
● Small and simple project

Issues?
● Flow - opt in , TS - have to write everything..
● 3rd party typings - But sometimes… those typings leave a lot to be desired
● TS - if things are not easily typed.. Maybe you didn’t structure your code well

Bad things...
● Type systems do not solve all of your mistakes, they only prevent some from happening
● They introduce complexity
● More programming knowledge is needed (Types? What are those?)
● You still need to know JS
● They don't make JS more performant
● Can lead to over-engineering
● Illusion of security (mistyped libraries, abusing any type)

● Elm - a Haskell inspired purely functional typed language that compiles down to JS
● Purescript - a Haskell inspired purely functional statically typed language that compiles down to JS
● Livescript - a Haskell, F# and clojure inspired functional statically typed language that also compiles
down to JS
● Dart - Google’s attempt at replacing JS - a more classical optionally-typed language that, you
guessed it, compiles down to JS
● Haxe - a strictly typed general purpose multi-paradigm language that also happens to also target JS
● All of that is great but it introduces new concepts, a new language and a new set of problems which
could be problematic for onboarding new people not familiar to them
Other typed solutions

REMIND ME AGAIN WHY
SHOULD I DO THIS?

Why?
● Refactoring
● Codebase robustness
● Helps with high people turnover rate
● Ease of development (code completion)
● Auto documentation - “what type is that second argument of a function from
over there?!?”
● Readability, static code analysis - catch errors early!

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