Why Static Type Checking is Better

WhyStaticTypeCheckingisBetter
@AndrewRota
JavaScript Engineer,

With JavaScript...anything is possible
@AndrewRota | #empirejs 2/48

Change all the types...
@AndrewRota | #empirejs
var x; // x starts undefined
typeof x === 'undefined'
x = 5; // now it's is a number
typeof x === 'number'
x = 'five'; // now it's a string
typeof x === 'string'
x = true; // now it's a boolean
typeof x === 'boolean'
x = {value: 'five'}; // now it's an object
typeof x === 'object'
JS
3/48

We have types, but they're not static.
(No type is known or declared at compile time)

7 Types in JavaScript
Boolean Number String Object
Symbol Null Undefined
5/48

Most code has type expectations
...we just don't always acknowledge it.
function add(arr) {
    var sum = 0;
    for (var i = 0; i < arr.length; i++) {
        sum += arr[i];
    }
    return sum;
}
add([2,3]); // returns 5 as expected
add(['foo', 'bar']);  // returns "0foobar" (expected?)
add(null);  // Throws exception: cannot read prop 'length' of null
JS
6/48

What to do?

Ad Hoc Runtime Checks
function add(arr) {
    if (!Array.isArray(arr)) return;
    var sum = 0;
        if (typeof arr[i] !== 'number') return;
        sum += arr[i];
    }
    return sum;
}
JS
8/48

Ad Hoc Runtime Checks
function add(arr) {
    if (!Array.isArray(arr)) throw('not an array');
    var sum = 0;
        if (typeof arr[i] !== 'number') throw('not a number');
        sum += arr[i];
    }
    return sum;
}
JS
9/48

Type Annotations
/**
* Add all numbers in an array
*
* @param {number[]}
* @return {number}
*/
function add(arr) {
    var sum = 0;
        sum += arr[i];
    }
    return sum;
}
JS
10/48

Type Annotations
Explicit typing...without any checks.
/**
*
* @param {number[]}
* @return {number}
*/
function add(arr) {
    var sum = 0;
        sum += arr[i];
    }
    return sum;
}
JS
11/48

Code Reviews !== Type Checks

Type Checking in JavaScript

ActionScript
Early 2000s | Partially conformed to ECMAScript 4
private function add(arr:Array):int{
    var sum:int = 0;
    for (var i:int = 0; i < arr.length; i++) {
        sum += arr[i];
    }
    return sum;
}
AS
14/48

Closure Compiler
~2009
/**
*
* @param {number[]}
* @return {number}
*/
function add(arr) {
    var sum = 0;
        sum += arr[i];
    }
    return sum;
}
JS
15/48

TypeScript
2012 | Compiles to JavaScript, optional static typing
function add(arr:Array<number>):number{
    var sum:number = 0;
    for (var i:number = 0; i < arr.length; i++) {
        sum += arr[i];
    }
    return sum;
}
TS
16/48

Flow
2014 | Compiles to JavaScript, optional static typing
/* @flow */
function add(arr:Array<number>):number{
    var sum:number = 0;
    for (var i:number = 0; i < arr.length; i++) {
        sum += arr[i];
    }
    return sum;
}
FLOW
17/48

Flow Comments
2015 | Alternative to a transpile step
/* @flow */
function add(arr/*:Array<number>*/)/*:number*/{
    var sum/*:number*/ = 0;
    for (var i/*:number*/ = 0; i < arr.length; i++) {
        sum += arr[i];
    }
    return sum;
}
JS
18/48

Released Runtime Env. No Transpile Null Checking ES6
Closure Compiler 2009 Java ✓ X Some
TypeScript 2012 JavaScript X X Some
Flow 2014 OCaml X ✓ Some
Flow Comments 2015 OCaml ✓ ✓ Some

Adding Gradual Type Checks

Step 1: Choose a Type Checker
Step 2: Set Up a Transpile Step
Step 3: Add Type Annotations

TypeScript Flow
TypeScript vs. Flow
Released 2012
Written in JS: any OS
Community-provided
declaration files
Addtional transpiled features
(defaults, overloads)
·
·
·
·
Released 2014
Written in OCaml: OSX, Linux
Built-in null handling.
Comment-only syntax
available
·
·
·
·
23/48

Setting Up Flow
1. Install Flow from flowtype.org
2. Add transformer ( JSX or Babel ) to your build

Using Flow
1. Run flow check
2. Run build with transformer.
Check → Transform
26/48

Using Flow

Setting Up TypeScript
Install TypeScript with npm i ‐g typescript

Using TypeScript
Run tsc myFile.ts

Using TypeScript

Type Inference
Some of your work's already done!
Flow: property length: Property not found in Number
TypeScript: Property 'length' does not exist on type 'number'
var x = 1;
x.length;
JS
32/48

Adding Basic Types
Flow: number, string, boolean, void, Array, Function, Object, mixed, any
TypeScript: number, string, boolean, void, Array, Function, Object, any, enum
var x:string = 'test'; TS/FLOW
33/48

Arrays
var list:number[] = [1,2,3];
var anotherList:Array<number> = [1,2,3];
TS/FLOW
34/48

Union Types
ThisType | ThatType
var x: number | string = 0;
x = 'foo';
TS/FLOW
35/48

Null Checks
Flow has the advantage here
Flow: property x: Property cannot be accessed on possibly null value
TypeScript: no error
var x = null;
x.foo;
JS
36/48

Functions
Both arguments and return values (the function itself)
function helloWorld(name: string):string {
return 'Hello, ' + name;
}
function addExclamation(sentence: string):string {
return sentence + '!';
}
addExclamation(helloWorld('EmpireJS'));
TS/FLOW
37/48

Object Literals
function doSomething(modelObject: {title: string}) {
return modelObject.title;
}
doSomething({title: 'My Object!', id: 2, flag: true});
TS/FLOW
38/48

Interfaces
External interfaces via declaration files
→ Find TypeScript declarations at definitelytyped.org
interface Model {
title: string
}
function doSomething(modelObject: Model) {
return modelObject.title;
}
doSomething({title: 'My Object!', id: 2, flag: true});
TS/FLOW
39/48

And there's more!
Classes
Modules
Nullable Types
Generics
Polymorphism
·
·
·
·
·
40/48

But is it worth it?

Catch More Bugs at Compile Time
Flow: null: This type is incompatible with string
function helloWorld(name:string) {
return 'Hello, ' + name;
}
helloWorld('EmpireJS');
helloWorld(null);

FLOW
42/48

Self-Document Code Behavior
function flatten (input, shallow, strict) {}

JS
function flatten (
    input: Array<any>,
    shallow: boolean,
    strict: boolean
): Array<any> {}

TS/FLOW
source: http://flowtype.org/docs/underscore.html#_
43/48

Easier to Reason About Code Flow
In: Number
Out: String
↗
In: String
Out: Object
↗
In: Object
Out: Boolean
44/48

- Bertrand Meyer
Correctness
“ The ability of software products to perform their exact tasks, as
defined by their specification.

Tooling
IntelliJ
Sublime

- TypeScript and the Road to 2.0
And someday it might be in JavaScript...
“ The TypeScript team is [...] looking forward to working together going
forward and creating the best tools we can for the JavaScript community.
In the long term, we will also be working to fold the best features of
these tools into ECMAScript, the standard behind JavaScript.

Thanks!
twitter @andrewrota
github github.com/andrewrota

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