Introduction to TypeScript. Interfaces

Basic syntax

We know that we can use object literals to specify what property types we can use. However, what if you have multiple objects with the same structure? Interface is a programming structure that allows to create types for objects.

interface Settings {
  color: string;
  delay: number;
  retry: boolean;
}

const mySettings: Settings = {
  color: "black",
  delay: 1000,
  retry: false
}

console.log(mySettings.color);

Pay attention that when we define interface we use "semicolon".

Optional property

We can define optional property using ?: symbols:

interface Settings {
  color: string;
  delay: number;
  retry?: boolean;
}

const mySettings: Settings = {
  color: "black",
  delay: 1000
}

console.log(mySettings.retry); //undefined

Index signatures

Sometimes we don't know how many properties an object will have. We can specify expected types:

interface Person {
  [key: string]: string;
}

const p: Person = {
  name: 'Mary',
  lastName: 'Smith'
}

p['age'] = 50; // Type 'number' is not assignable to type 'string'

Methods

Of course, we should be able to define methods:

interface Settings {
  color: string;
  delay: number;
  describe: () => void;
}

const mySettings: Settings = {
  color: "black",
  delay: 1000,
  describe: function () {
    console.log("Your favorite color is " + this.color)
  }
}

console.log(mySettings.describe());

Here, in interface, we defined type of describe as () => void, that means function. In this case void doesn't indicate that a function mustn't return any value, it indicates that it can return any value. The other way to set function type is : describe(): void.

Inheritance

Interfaces can inherit properties from another ones using extends:

interface Settings {
  color: string;
  delay: number;
  describe: () => void;
}

interface Game extends Settings{
  character: string;
}

const myGame: Game = {
  color: "black",
  delay: 1000,
  describe: function () {
    console.log("Your favorite color is " + this.color);
    return 'hi';
  },
  character: 'goat'
}

console.log(myGame.character); // goat

Using as argument in function

interface Settings {
  color: string;
  delay: number;
}

function changeSettings(s: Settings): Settings {
  return {
    color: s.color,
    delay: s.delay + 1000
  }
}

We specify what we expect the argument to be.