7种原始类型
    Boolean
    Null
    Undefined
    Number
    BigInt
    String
    Symbol
    和Object
    共8种类型

    1. let isDone: boolean = false
    2. let age: number = 23
    3. let binaryNumber: number = 0b1111
    5. let firstName: string = 'kip'
    7. let message: string = `hello,${firstName},age is ${age}`
    9. let u: undefined = undefined
    10. let n: null = null
    12. let num: number = undefined