Modern Typescript with Examples Cheat Sheet
Typing Objects
Mapped Types - Getting Types from Data
- Using typeof / keyof
- Using keyof with Generics and Interfaces
Immutability
Strict Mode
- Non-Nullable Types
- Strict Class Property Initialization
Unknown Type
- Example: Reading JSON from localStorage
Generics
Discriminated Unions
- Exhaustive Pattern Matching Using never
Optional Chaining
- ?. returns undefined when hitting a null or undefined
Nullish Coalescing
- ?? “fall Backs” to a Default Value When Dealing with null or undefined
Assertion Functions

Modern Typescript with Examples Cheat Sheet

Typing Objects

Property, Method, Index, Call and Construct signatures

interface ExampleInterface {
  // Property signature
  myProperty: boolean;
  callback: MyFunctionType;

  // Method signature
  myMethod(x: string): void; // parameters (x) document how things work, but have no other purpose

  // Index signature
  [prop: string]: any; // help describe Arrays or objects that are used as dictionaries

  // Call signature
  (x: number): string; // enable interfaces to describe functions

  // Construct signature
  new (x: string): ExampleInstance; // enable describing classes and constructor functions
}

Index Signature Additional Rules

If there are both an index signature and property and/or method signatures in an interface, then the type of the index property value must also be a supertype of the type of the property value and/or method.

interface I1 {
  [key: string]: boolean;

  //@ts-ignore: Property 'myProp' of type 'number' is not assignable to string index type 'boolean'.(2411)
  myProp: number;

  //@ts-ignore: Property 'myMethod' of type '() => string' is not assignable to string index type 'boolean'.(2411)
  myMethod(): string;
}

interface I2 {
  [key: string]: number;
  myProp: number; // NO errors
}

Using A Construct Signature

A class has two types: the type of the static side and the type of the instance side. The constructor sits in the static side, when a class implements an interface, only the instance side of the class is checked.

interface ClockInterface {
  tick(): void;
}
interface ClockConstructor {
  new (hour: number, minute: number): ClockInterface;
}

// Using Class Expression
const ClockA: ClockConstructor = class Clock implements ClockInterface {
  constructor(h: number, m: number) {}
  tick() {}
};

let clockClassExpression = new ClockA(18, 11);

// Using Class Declaration with a Constructor Function
class ClockB implements ClockInterface {
  constructor(h: number, m: number) {}
  tick() {}
}

function createClock(
  ctor: ClockConstructor,
  hour: number,
  minute: number
): ClockInterface {
  return new ctor(hour, minute);
}

let clockClassDeclaration = createClock(ClockB, 12, 17);

Type Literal Syntax

Typically used in the signature of a higher-order function

type MyFunctionType = (name: string) => number;

Optional `?` and `readonly` Properties

interface Name {
  readonly first: string;
  middle?: string;
  last: string;
}

Excess Properties (⛔ Inconsistency)

TypeScript is a structurally typed language. This means that to create a Dog you don’t need to explicitly extend the Dog interface. Instead any object with a breed property that is of type string can be used as a Dog (see example below).

Engineers can’t just think of interfaces as “objects that have exactly a set of properties” or “objects that have at least a set of properties”.

Inline object arguments receive an additional level of validation that doesn’t apply when they’re passed as variables.

interface Dog {
  breed: string;
}

function printDog(dog: Dog) {
  console.log("Dog: " + dog.breed);
}

const ginger = {
  breed: "Airedale",
  age: 3
};

printDog(ginger); // excess properties are OK!

printDog({
  breed: "Airedale",
  age: 3
});
// excess properties are NOT OK!! Argument of type '{ breed: string; age: number; }' is not assignable..

Mapped Types - Getting Types from Data

Using `typeof` / `keyof`

const data = {
  value: 123,
  text: "text",
  subData: {
    value: false
  }
};
type Data = typeof data;
// type Data = {
// value: number;
// text: string;
// subData: {
//   value: boolean;
//
}

const data = ["text 1", "text 2"] as const;
type Data = typeof data[number];
// type Data = "text 1" | "text 2"

const locales = [
  {
    locale: "se",
    language: "Swedish"
  },
  {
    locale: "en",
    language: "English"
  }
] as const;
type Locale = typeof locales[number]["locale"];
// type Locale = "se" | "en"

const currencySymbols = {
  GBP: "£",
  USD: "$",
  EUR: "€"
};
type CurrencySymbol = keyof typeof currencySymbols;
// type CurrencySymbol = "GBP" | "USD" | "EUR"

Using `keyof` with Generics and Interfaces

interface HasPhoneNumber {
  name: string;
  phone: number;
}

interface HasEmail {
  name: string;
  email: string;
}

interface CommunicationMethods {
  email: HasEmail;
  phone: HasPhoneNumber;
  fax: { fax: number };
}

function contact<K extends keyof CommunicationMethods>(
  method: K,
  contact: CommunicationMethods[K] // 💡turning key into value -- a *mapped type*
) {
  //...
}
contact("email", { name: "foo", email: "mike@example.com" });
contact("phone", { name: "foo", phone: 3213332222 });
contact("fax", { fax: 1231 });

// // we can get all values by mapping through all keys
type AllCommKeys = keyof CommunicationMethods;
type AllCommValues = CommunicationMethods[keyof CommunicationMethods];

Immutability

`readonly` Array / Tuple

const array: readonly string[];
const tuple: readonly [string, string];

`readonly` Properties

Properties marked with readonly can only be assigned to during initialization or from within a constructor of the same class.

type Point = {
  readonly x: number;
  readonly y: number;
};

const origin: Point = { x: 0, y: 0 }; // OK
origin.x = 100; // Error

function moveX(p: Point, offset: number): Point {
  p.x += offset; // Error
  return p;
}

function moveX(p: Point, offset: number): Point {
  // OK
  return {
    x: p.x + offset,
    y: p.y
  };
}

`readonly` Class Properties

Gettable area property is implicitly read-only because there’s no setter:

class Circle {
  readonly radius: number;

  constructor(radius: number) {
    this.radius = radius;
  }

  get area() {
    return Math.PI * this.radius ** 2;
  }
}

`const` Assertions

// Type '10'
let x = 10 as const;

array literals become readonly tuples

// Type 'readonly [10, 20]'
let y = [10, 20] as const;

object literals get readonly properties
no literal types in that expression should be widened (e.g. no going from "hello" to string)

// Type '{ readonly text: "hello" }'
let z = { text: "hello" } as const;

⛔ const contexts don’t immediately convert an expression to be fully immutable.

let arr = [1, 2, 3, 4];

let foo = {
  name: "foo",
  contents: arr
} as const;

foo.name = "bar"; // Error!
foo.contents = []; // Error!

foo.contents.push(5); // ...works!

Strict Mode

Non-Nullable Types

--strictNullChecks

let name: string;
name = "Marius"; // OK
name = null; // Error
name = undefined; // Error

let name: string | null;
name = "Marius"; // OK
name = null; // OK
name = undefined; // Error

Optional parameter ? automatically adds | undefined

type User = {
  firstName: string;
  lastName?: string; // same as `string | undefined`
};

In JavaScript, every function parameter is optional, when left off their value is undefined. We can get this functionality in TypeScript by adding a ? to the end of parameters we want to be optional. This is different from adding | undefined which requires the parameter to be explicitly passed as undefined

function fn1(x: number | undefined): void {
  x;
}

function fn2(x?: number): void {
  x;
}

fn1(); // Error
fn2(); // OK
fn1(undefined); // OK
fn2(undefined); // OK

Type guard needed to check if Object is possibly null

function getLength(s: string | null) {
  // Error: Object is possibly 'null'.
  return s.length;
}

function getLength(s: string | null) {
  if (s === null) {
    return 0;
  }

  return s.length;
}

// JS's truthiness semantics support type guards in conditional expressions
function getLength(s: string | null) {
  return s ? s.length : 0;
}

function doSomething(callback?: () => void) {
  // Error: Object is possibly 'undefined'.
  callback();
}

function doSomething(callback?: () => void) {
  if (typeof callback === "function") {
    callback();
  }
}

Strict Class Property Initialization

class User {
  // Type error: Property 'username' has no initializer
  // and is not definitely assigned in the constructor
  username: string;
}

class User {
  username: string | undefined;
}

const user = new User();

// Whenever we want to use the username property as a string, though, we first have to make sure that it actually holds a string and not the value undefined
const username =
  typeof user.username === "string" ? user.username.toLowerCase() : "n/a";

class User {
  username = "n/a";
}

const user = new User();

// OK
const username = user.username.toLowerCase();

class User {
  constructor(public username: string) {}
}

const user = new User("mariusschulz");

// OK
const username = user.username.toLowerCase();

Unknown Type

Example: Reading `JSON` from `localStorage`

type Result =
  | { success: true; value: unknown }
  | { success: false; error: Error };

function tryDeserializeLocalStorageItem(key: string): Result {
  const item = localStorage.getItem(key);

  if (item === null) {
    // The item does not exist, thus return an error result
    return {
      success: false,
      error: new Error(`Item with key "${key}" does not exist`)
    };
  }

  let value: unknown;

  try {
    value = JSON.parse(item);
  } catch (error) {
    // The item is not valid JSON, thus return an error result
    return {
      success: false,
      error
    };
  }

  // Everything's fine, thus return a success result
  return {
    success: true,
    value
  };
}

Generics

`<T>` With and Without Type Argument Inference

function identity<T>(arg: T): T {
  return arg;
}

let output = identity<string>("myString"); // type of output will be 'string'
let output = identity("myString"); // The compiler sets the value of `T` based on the type of the argument we pass in

`<F, S>` - Using More Than One Type Argument

No value arguments are needed in this case.

function makePair<F, S>() {
  let pair: { first: F; second: S };

  function getPair() {
    return pair;
  }

  function setPair(x: F, y: S) {
    pair = {
      first: x,
      second: y
    };
  }
  return { getPair, setPair };
}

// Creates a (number, string) pair
const { getPair, setPair } = makePair<number, string>();
// Must pass (number, string)
setPair(1, "y");

Higher Order Function with `Parameters<T>` and `ReturnType<T>`

// Input a function `<T extends (...args: any[]) => any>`
// Output a function with same params and return type `:(...funcArgs: Parameters<T>) => ReturnType<T>`
function logDuration<T extends (...args: any[]) => any>(func: T) {
  const funcName = func.name;

  // Return a new function that tracks how long the original took
  return (...args: Parameters<T>): ReturnType<T> => {
    console.time(funcName);
    const results = func(...args);
    console.timeEnd(funcName);
    return results;
  };
}

function addNumbers(a: number, b: number): number {
  return a + b;
}
// Hover over is `addNumbersWithLogging: (a: number, b: number) => number`
const addNumbersWithLogging = logDuration(addNumbers);

addNumbersWithLogging(5, 3);

Discriminated Unions

The code doesn’t compile if you don’t cover every possibility: this is what gives you power. If you can expose your types as a common interface, then using OO features (interfaces/polymorphism) will make your life better by putting type-specific behaviour in the type rather than in the consuming code.

It is important to recognise that interfaces and unions are kind of the opposite of each other: an interface defines some stuff the type has to implement, and the union defines some stuff the consumer has to consider. If you add a method to an interface, you have changed that contract, and now every type that previously implemented it needs to be updated. If you add a new type to a union, you have changed that contract, and now every exhaustive pattern matching over the union has to be updated. They fill different roles, and while it may sometimes be possible to implement a system ‘either way’, which you go with is a design decision: neither is inherently better.

Exhaustive Pattern Matching Using `never`

interface Square {
  kind: "square";
  size: number;
}
interface Rectangle {
  kind: "rectangle";
  width: number;
  height: number;
}
interface Circle {
  kind: "circle";
  radius: number;
}
interface Triangle {
  kind: "triangle";
  whatever: number;
}

type Shape = Square | Rectangle | Circle | Triangle;

function assertNever(x: never): never {
  throw new Error("Unexpected object: " + x);
}

function area(s: Shape) {
  switch (s.kind) {
    case "square":
      return s.size * s.size;
    case "rectangle":
      return s.height * s.width;
    case "circle":
      return Math.PI * s.radius ** 2;
    default:
      return assertNever(s); // Argument of type 'Triangle' is not assignable to parameter of type 'never'.
  }
}

Optional Chaining

`?.` returns `undefined` when hitting a `null` or `undefined`

Album where the artist, and the artists bio might not be present in the data.

type AlbumAPIResponse = {
  title: string;
  artist?: {
    name: string;
    bio?: string;
    previousAlbums?: string[];
  };
};

// Instead of:
const maybeArtistBio = album.artist && album.artist.bio;

// ?. acts differently than the &&s since && will act differently on "falsy" values (e.g. an empty string, 0, NaN, and false).
const artistBio = album?.artist?.bio;

// optional chaining also works with the [] operators when accessing elements
const maybeArtistBioElement = album?.["artist"]?.["bio"];
const maybeFirstPreviousAlbum = album?.artist?.previousAlbums?.[0];

Nullish Coalescing

`??` “fall Backs” to a Default Value When Dealing with `null` or `undefined`

Value foo will be used when it’s “present”; but when it’s null or undefined, calculate bar() in its place.

let x = foo ?? bar();

// instead of

let x = foo !== null && foo !== undefined ? foo : bar();

It can replace uses of || when trying to use a default value, and avoids bugs. When localStorage.volume is set to 0, the page will set the volume to 0.5 which is unintended. ?? avoids some unintended behaviour from 0, NaN and "" being treated as falsy values.

function initializeAudio() {
  let volume = localStorage.volume || 0.5; // Potential bug
}

Assertion Functions

Assertions in JavaScript are often used to guard against improper types being passed in.

A Standard JavaScript `Assert()` Doesn’t Work for Type Checking

function yell(str) {
  assert(typeof str === "string");

  return str.toUppercase();
  // Oops! We misspelled 'toUpperCase'.
  // Would be great if TypeScript still caught this!
}

Using `if` and `typeof` Everywhere is Bloat

function yell(str) {
  if (typeof str !== "string") {
    throw new TypeError("str should have been a string.");
  }
  // Error caught!
  return str.toUppercase();
}

Assertion Function Style 1 - Check for a Condition

function assert(condition: any, msg?: string): asserts condition {
  if (!condition) {
    throw new AssertionError(msg);
  }
}

function yell(str) {
  assert(typeof str === "string");

  return str.toUppercase();
  //         ~~~~~~~~~~~
  // error: Property 'toUppercase' does not exist on type 'string'.
  //        Did you mean 'toUpperCase'?
}

Assertion Function Style 2 - Tell Typescript That a Specific Variable or Property Has a Different Type

Very similar to writing type predicate signatures.

function assertIsString(val: any): asserts val is string {
  if (typeof val !== "string") {
    throw new AssertionError("Not a string!");
  }
}

function yell(str: any) {
  assertIsString(str);

  // Now TypeScript knows that 'str' is a 'string'.

  return str.toUppercase();
  //         ~~~~~~~~~~~
  // error: Property 'toUppercase' does not exist on type 'string'.
  //        Did you mean 'toUpperCase'?
}

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Modern Typescript with Examples Cheat Sheet

Typing Objects

Property, Method, Index, Call and Construct signatures

Index Signature Additional Rules

Using A Construct Signature

Type Literal Syntax

Optional ? and readonly Properties

Excess Properties (⛔ Inconsistency)

Mapped Types - Getting Types from Data

Using typeof / keyof

Using keyof with Generics and Interfaces

Immutability

readonly Array / Tuple

readonly Properties

readonly Class Properties

const Assertions