01_2_types-string-number-boolean-union.md

Types: String, Number, Boolean and Union Types

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• String
• Number
• Boolean
• Union Types (aka, OR)
  • Discriminated unions
• Intersection Types (aka, AND)

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String

let name: string;
name = 'Peter';

Number

Including ALL integers (-33, 33) and float (3.3)

let age: number;
age = 33;

Boolean

let completed: boolean;
completed = false;

Union Types

(aka, OR) When more than one type can be used.

let zipCode: number | string;
zipCode = 94115;
zipCode = '94115';

We can narrow with type guards (known also as type narrowing):

1. === or !===
2. typeof
3. instaceof
4. property in object

Example:

function parseZipCode(zip: number | string) {
  if (typeof zip !== 'string') return zip.toString();
  return zip;
}

console.log(parseZipCode(94115)); // "94115" 
console.log(parseZipCode('94115')); // "94115" 

Discriminated unions

Taking the previous example, we add a property (example: kind) to all the types in the union (in this case Cat and Fish) Then, we check the value of kind

type Animal = Cat | Fish;

type Cat = {
  kind: 'cat',
  walk: Function
}

type Fish = {
  kind: 'fish'
  swim: Function
}

const cat: Cat = {
  kind: 'cat',
  walk: function(): void {
    console.log(`walking`);
  }
}

const fish: Fish = {
  kind: 'fish',
  swim: function(): void {
    console.log(`swimming`);
  }
}

function move(animal: Animal) {
  if (animal.kind === 'cat') {
    animal.walk();
  }
  if (animal.kind === 'fish') {
    animal.swim();
  }
}

move(cat); // walking
move(fish); // swimming

Intersection Types

(aka, AND)

let user: { name: string; age: number } & { username: string }

user = {
    name: 'Peter',
    age: 33,
    username: 'peter'
}

If we try to define a user without username, TS will complain with:

Type '{ name: string; age: number; }' is not assignable to type '{ name: string; age: number; } & { username: string; }'.
  Property 'username' is missing in type '{ name: string; age: number; }' but required in type '{ username: string; }'.