Flutter for JavaScript Developers

So you’re a web developer working with JavaScript and curious about Flutter? That was me about one-year ago, and at the time, learning an uncommon language like Dart seemed ridiculous because there are existing hybrid app frameworks in JS. Then I tried it… To my surprise, I was able to write productive code right out of the gate. The following lesson will compare Dart/Flutter to JS/ReactNative.

The Tradeoffs

There are no solutions; there are only trade-offs. - Thomas Sowell

Flutter is awesome 🐦🤟 because it…

• has amazing docs & tooling
• performs faster because it has no JavaScript bridge
• can integrate native code
• hot reloads
• small code footprint and no import/export/bundling headaches
• it positions you for future development with Fuchsia

Flutter is meh 😒 because it…

• has a steeper learning curve with Dart
• cannot be debugged in the browser
• has a small ecosystem of developers and plugins relative to JS

Dart vs JS - Syntax

Dart is a strongly-typed compiled language, while JS is a weakly-typed scripting language. Despite being polar opposites in these ways, they share many of the same convetions, so most JS devs will be productive on day one.

Variables

• SomeType foo; 👉 let foo
• final 👉 const
• const 👉 Object.freeze(obj)

// Reassignable variables
let person: string;
person = 'jeff';
person = 'bob';

// Single assignment variables
const lucky: number = 23;

// Implicit
const implicit = { name: 'Jeff' }; 

// Any
const whoKnows: any = getSomethingDynamic();

// Reassignable variables
String person;
person = 'Jeff';
person = 'Bob';

// Single Assignment
final int lucky = 23;

// Single Assignment & Immutable
const Map person = 'Jeff';

// Implicit
var implicit = { 'name': 'Jeff' }; // Map<String, String>

// Any type
dynamic whoKnows = getSomethingDynamic();

Imports/Exports

In JS, you have several ways to import/export values, so let’s just look at the modern ES6 way.

export default const = 'JS';
// or
export const lang = 'JS';

import lang from 'globals';
// or
import { lang } from 'globals';

When you import a file in Dart, you will have access to everything declared that file. It looks trivial here, but it eliminates the import hell 😠 that you find in basically every big JS project.

final String lang = 'Dart!';

import 'globals.dart';

main() {
  print(lang)
}

Logging

console.log('howdy!');

print('howdy!');

Entry Point

JS doesn’t have a required entry point to start executing our code, but it’s common to export a function.

export const app = () => {
    // init app
};

app();

In dart, a main function is required.

main() {

}

Maps and Objects

• Map 👉 cross between Object and Map

Dart has a Map class used to represent key-value pairs. It combines some of the features from in the JS Object & Map.

const state = { lucky: 23 }
state['unlucky'] = 11;

const state = new Map([['lucky', 23]]);
state.set('unlucky', 11);

Map state = { 'lucky': 23 };
state['unlucky'] = 11;

Lists and Arrays

• List 👉 Array

A Dart List is similar to a JS Array, containing most of the same instance and class methods (and beyond).

const things: number[] = [1, 2, 3]

List<int> things = [1, 2, 3];

things.forEach()
things.reduce()
things.last;

Functions

Functions in Dart should feel very familiar to the JS dev - it supports named, anonymous, arrow, and higher-order functions. The main difference is that you don’t need the function keyword.

function addOne(val: number) {
    return val + 1;
}

// Higher Order
function callme(cb: Function) {
    return cb('Hello?');
}

callme((v) => console.log(v));

addOne(int val) {
    return val + 1;
} 

// Higher Order
callme(Function cb) {
    return cb('Hello?');
}

callme((v) => print(v));

Classes

You will be using Classes frequently in Dart and they are very powerful. The language supports mixin-based inheritance, which provides excellent code reuse with composition. In JS, classes are just syntatic sugar for functions and prototypal inheritance.

export const main = () => {
    const myComponent = new Component()
}

class Component { }

Defining and instantiating a class is nearly identical, but Dart does not require the new keyword.

main() {
  var myWidget = Widget();
}

class Widget { }

Class Constructors

In Dart, you can construct a class by calling its name like a Function. In addition, you can use named constructors to instantiate the same with different logic.

export const main = () => {
    const bar = new Component(1,2);
}

class Component { 
    constructor(private a, public b) {
        // non-ts version
        // this.a = a
        // this.b = b
    }
}

main() {
  Widget foo = Widget.withStrings('1', '2');
}

class Widget { 

  // Default Constructor
  Widget(_a, b);

  // Named Constructor
  Widget.withStrings(_a, b);
  
}

Async Programming

Futures and Promises

• Future 👉 Promise

A Dart Future is almost identical to a JS Promise, which is really nice because this is one of the more complex concepts master.

async function howdy() {
    return 'partner 🤠';
}

async function greet() {
    await howdy();
}

greet().then(console.log).catch()

Future howdy() async {
    return 'partner 🤠';
}

greet() async {
    await howdy();
}

greet().then(print).catchError();

Streams and Observables

• Stream 👉 Observable

JavaScript does not have a native stream-like data structure yet, but they are commonly provided via supporting packages like RxJS, Mobx, and others. A stream is similar to a Promise (Future), except that is

A Stream is like a Future that emits multiple values over time and we can even use it with async/await 😍.

main() async {

  var items = Stream.fromIterable([1,2,3]);

  await for (int i in items) {
    // do something 
  }

  // Or setup a listener
  items.listen(print);
}

There is one big concept to keep in mind, single subscription streams (default) will throw an error if multiple subscribers try to listen.

• Single-subscription streams 👉 Cold observable
• Broadcast streams 👉 Hot Observable

Even though streams are available in Dart, I often find myself using RxDart in Flutter. The beauty of RxDart is that it builds on top of streams, rather than try to reinvent the wheel, so anything that uses a stream in Flutter can also use an Rx observable. Also, if you’re coming from the Angular world you can keep the operators you know and love like switchMap, takeUntil, scan, and so on.

Flutter vs Frameworks

Flutter is inspired by JavaScript frameworks like React/Angular/Vue that enable components to reactively update the UI when data changes. In fact, Flutter provides a setState method that works exactly like it does in ReactJS.

Overall, it is not very opinionated about how architect the app. You can use composition to build a bunch of tiny widgets in their own files, or create a few big widgets in a single file.

Dependency Management

• pubspec.yaml 👉 package.json
• Pub 👉 NPM

Dependencies are registered in the pubspec.yaml and should auto-update on save.

Widgets and Components

• Widget 👉 Component
• build 👉 render

As you may have guessed, Flutter has components similar to those in React, with a build method that can be called when state changes.

import React from 'react';
import { StyleSheet, Text, View } from 'react-native';

export default class MyApp extends React.Component {
  render() {
    return (
      <View>
        <Text>Hello world!</Text>
      </View>
    );
  }
}

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  build(context) {
    return Center(
      child: Text('Hello World'),
    );
  }
}

Full React Native Comparison

Let’s go ahead and recreate this styled component from the react native docs. You’ll notice the Flutter version on the left has about 65% of the code footprint, with a guarantee of type-safety. Also see the other examples in this gist.

Flutter provides several ways to manage component state. You can use setState just like React does, but there are some additional built-in techniques. My personal favorite is to represent stateful data as a Stream or Rx Observable, then use the StreamBuilder widget to conditionally paint the UI. As you can see, this does not require any explicit calls to render the UI, rather it’s based on the most recent value emitted from the stream. The code footprint is improved slightly, but this gains really add up if you find yourself setting the state frequently.