変数に Type Annotations （型注釈）をつける方法 - TypeScript

let apples: number = 5;
let speed: string = 'fast';
let hasName: boolean = true;

let nothingMuch: null = null;
let nothing: undefined = undefined;

let now: Date = new Date();

let colors: string[] = ['red', 'green', 'blue'];
let myNumbers: number[] = [1, 2, 3];
let truths: boolean[] = [true, true, false];

class Car {}
let car: Car = new Car();

let point: { x: number; y: number } = { // y の後にセミコロンは要らない
  x: 10,
  y: 20
};

const logNumber: (i: number) => void = (i: number) => {
  console.log(i);
};