string 字符串
    number 数值
    NaN
    Infinity
    boolean true / false
    void undefined
    null null
    undefined undefined
    symbol Symbol()

    默认模式下可以为 null 

    1. const a: string = 'string';
    2. const b: number = 1;
    3. const c: boolean = false;
    4. const d: void = undefined;
    5. const e: null = null;
    6. const f: undefined = undefined;
    7. const g: symbol = Symbol();