TypeScript 001: Basic Types

最近开始学习TypeScript的一些随手笔记，年纪大了怕忘了。

Basic Types

• Boolean

  let isDone: boolean = false;

• Number

  let decimal: number = 6;

• String

  let fullName: string = `Bob Dylan`
  let sentence: string = "Hello, my name is " + fullName + ".\n\n" + "I'll be " + (age + 1) + " years old next month."

• Array

  let list: number[] = [1, 2, 3];
  let list: Array<number> = [1, 2, 3];

• Tuple

  • array with a fixed number of elements whose types are known

  let x: [string, number]    // order matters

• Enum

  • enums begin numbering their members starting at 0
  • you can go from a numeric value to the name of that value in the enum

  enum Color {Red, Green, Blue}
  let c: Color = Color.Green;

  enum Color {Red = 1, Green, Blue}
  let colorName: string = Color[2]
  
  console.log(colorName);   // Green

• Any

  • describe the type of variables that we do not know when we are writing an application

  let notSure: any = 4;
  notSure = "maybe a string instead"
  notSure = false;

• Void

  • the absence of having any type at all

  function warnUser(): void {
    console.log("This is my warning message")
  }

• Null and Undefined

  let u: undefined = undefined;
  let n: null = null;

• Never

  • represents the type of values that never occur
  • the return type for a function expression that always throws an exception or one that never returns
  • the never type is a subtype of every type
  • no type is a subtype of never

  function error(message: string): never {
    throw new Error(message)
  }

• Object

  declare function create(0: object | null): void;
  
  create({ prop: 0 });

• Type assertions

  let someValue: any = "this is a string";
  let strLength: number = (<string>someValue).length

  let someValue: any = "this is a string";
  let strLength: number = (someValue as string).length