1. var str:string = "hello world";
    2. str = 124;
    3. console.log(str); //error
    1. number,string,boolean,null,undefined
    1. var str:string = "hello world";
    2. var num:number = 123;
    3. var b:boolean = true;
    4. b = false;
    5. var flag:null = null;
    6. var test:undefined = undefined;