Introduction to TypeScript basic types

;(function () {
  /*
   * In TS, you can define data types (type annotations) by using let variable name: data type = variable value.
   * You can also not specify the data type when defining it, TS will infer the data type itself*/
  // Boolean type let boo: boolean = false // Assigning a non-boolean value will throw an exception // Number type let num: number = 100

  // string let str: string = 'string' // Use single or double quotes to position str = `template string` // Use template string definition // Any type -> indicates that the type can be a dynamic type, which removes type checking at compile time let AnyType: any = 123
  AnyType = true // Duplicate assignment will not throw an exception // Void type -> Usually used for function types without return values ​​function demo(): void {
    console.log('test void type')
  }
  demo()

  // There are two special types, null and undefined
  // These two types are subtypes of all types, which means that these two types can be assigned to types such as number and string. let u: undefined = undefined
  num = u // Assign the value of the number type variable to undefined
  let n: null = null
  boo = n // Assign a boolean variable to null
})()

;(function () {
  // Define an array type that only contains numbers let arr1: number[] = [1, 2, 3]
  console.log(arr1)
  // Define an array that can hold numbers, strings, and boolean values ​​let arr2: (number | string | boolean)[] = ['1', '2', true]
  console.log(arr2)
  // Define an array of any type let arr3 = [1, ['1', '2', true], true]
  console.log(arr3)

  // Define an array of object type. The object must have two properties: name and age. const objectArray: { name: string; age: number }[] = [
    { name: 'Yiwan Zhou', age: 18 },
  ]
  // Or declare it through type alias // Define a type alias through type type User = { name: string; age: number }
  const objectArr: User[] = [{ name: '一碗周', age: 18 }]
})()

;(function () {
  // Define a tuple whose values ​​are string and number respectively let tuple: [string, number] = ['123', 123]
  console.log(tuple) // [ '123', 123 ]
  // Assign tuple[0] = 'string' by index
  console.log(tuple) // [ 'string', 123 ]
  // Assign other types // tuple[0] = true
  // console.log(tuple) // throws an exception })()

// Nested tuples and arrays let tuples: [string, number][] = [
    ['123', 123],
    ['456', 456],
]

;(function () {
  // Define an object with two properties: MyName and age, where MyName is of string type and age is of number type. let obj: {
    MyName: string
    age: number
  }
  // Object assignment. If the assignment is not performed according to the type specified above, an exception will be thrown. obj = {
    MyName: 'Yiwan Zhou',
    age: 18,
  }
  console.log(obj) // { MyName: '一碗周', age: 18 }
})()

;(function () {
  let myName = 'A bowl of Zhou'
  myName = true // Error: cannot assign type 'boolean' to type 'string'
})()

let arr = [1, '2'] // Define an array containing strings and numbers // Reassign values ​​to the array defined above // ​​arr = [true, false] // Error cannot assign type 'boolean' to type 'string | number'

// There is also the following example let val = arr.length === 0 ? 0 : 'The array length is not 0'
// val = false // Error cannot assign type 'boolean' to type 'string | number'

;(function () {
  // Define an interface interface Person {
    name: string
    age: number
  }
  // Define an array through the interface defined above let arr: Person[] = [{ name: '一碗周', age: 18 }]
  // Traverse the defined array arr.forEach(item => {
    // Based on the current environment, the compiler automatically infers that item is of type hobby and does not have the hobby attribute console.log(item.hobby) // The attribute "hobby" does not exist on type "Person"
  })
})()

// If type information is specified in the context, the context will be ignored.
arr.forEach((item: any) => {
    // Based on the current environment, the compiler automatically infers that item is of type hobby and does not have the hobby attribute console.log(item.hobby) // The attribute "hobby" does not exist on type "Person"
})

let SomeValue: any = 'this is a string'
// Syntax 1 let StrLength1: number = (<string>SomeValue).length
// Syntax 2 as syntax let StrLength2: number = (SomeValue as string).length