Typescript

Documentation

Cheat Sheets

Exercism Notes

To start, run exercism configure --token=exercism configure --token=<your-api-token> and then download using exercism download --exercise=<exercise-slug> --track=<track-slug>
each time you download you have to run yarn install in the directory
To test the functionality, you need to change the type to commonjs in the package.json - described here and probably here
To run: npx tsc *.ts; node <filename.js>
To submit: exercism submit <implementation_file_paths>

Installing

npm install -g typescript
npx tsc --init to generate the tsconfig.json
for mocha, jest, chai etc you need to install the types: npm i @types/mocha, also this page suggests adding them to types in tsconfig.json
npx tsc <filename> will convert a .ts to a .js
For some reason I got errors with require on vscode that had to be resolved by:
- npm i -D @types/node
- Adding node to the types in tsconfig.json

Syntax	Description
Header	Title
Paragraph	Text

this is <html>code

Skeleton

Files:

.nvmrc
index.ts
package.json containing typescript

commands:

echo v14.4.0 > .nvmrc
nvm use
npm init
- test command: mocha if you want to then run npm install mocha
- npm install chai
npm install typescript
npx tsc <filename>

hello-world.test.ts:

import { helloworld } from './helloworld'
import { expect  } from 'chai'

describe('helloworld', () => {
    it('should return hi there', () => {
        expect(helloworld()).to.equal('hi there')
    })
})

hello-world.ts:

export const hithere = () : string => {
    return "hi there"
}

Comparison - Javascript and Typescript

Description of differences between JS and TS

TypeScript is a syntactical superset of JavaScript and adds optional static typing to the language. TS transpiles to JS.

TS	JS
Static Typing: types and interfaces	Dynamically typed
Object Oriented	Prototype-Based Object Orientation
Transpiles to JS	Just-In-time Compilation

Summary of Typescript features

Primitive types: number,boolean,string
Data types: Arrays, Enums, void
Additional Data types: Tuple, Union, never and any
Type annotations can be captured in separate declaration files .d.ts
Supports generics: function id<T>(x: T): T {
Union ypes: function successor(n: number | bigint): number

Types

interfaces vs types
I'm not 100% what the difference between a type and class definition is.

type Heading = {
    text: string;
    subheadings: Array<Heading>;
}

const heading: Heading = {text: "Welcome to Akalabeth, world of doom!", subheadings: []}

Intersection Types allow one type to be an instance of others

Type Declaration Files

Declared in a separate .d.ts file. Similar to a header file from C

declare namespace arithmetics {
    add(left: number, right: number): number;
    subtract(left: number, right: number): number;
    multiply(left: number, right: number): number;
    divide(left: number, right: number): number;
}

Tuples

Tuples can have two values of different types: var employee: [number, string] = [1, "Steve"];
You can have an array of tuples: var employee: [number, string][];

Hashes/Dictionary are created by using index signatures

There isn't the same as a javascript object which can take obj['i'] - this is covered by interfaces, in object index signatures

interface nucleoTideMap { [index: string]: string }
const dnaToRna: nucleoTideMap = { 'g':'c','c':'g','t':'a','a':'u'}

ADTs in Typescript

Good talk on ADTs - using options
Option (fp-ts)
an option has a .map function whereas map on an array works on every element, on an option it only operates on the value if the value is not null/undefined
also fold which operates only on a value, or
getOrElse which supplies a default value

Functions

VarArgs - variable length arguments

https://www.damirscorner.com/blog/posts/20180216-VariableNumberOfArgumentsInTypescript.html

Classes

TS supports ECMAscript 2015 classes syntax.

class ABC {
    name: string;
}
const abc = new ABC()