JavaScript_数据类型转换

JavaScript_数据类型转换

问题

• JavaScript是数据类型有哪些?
• parseInt() parseFloat() Number()转换的区别是什么?
• != !==的区别是什么?

---

parseInt() parseFloat() Number()

   
const a = 12.35;
console.log(parseInt(a));   //12

const a1 = '12.7as34';
console.log(parseInt(a1));  //12
console.log(parseFloat(a1));    //12.7


const b = null;
console.log(Number(b)); //0

const c = undefined;
console.log(Number(c)); //NaN

String() Boolean()

console.log(typeof String(123));    //string
console.log(String(3.1415926e2));  //314.15926
console.log(String(null));  //null
console.log(String(undefined));     //undefined
console.log(String.fromCharCode(97));   //a


console.log(Boolean(0.123));    //true
console.log(Boolean(0));        //false
console.log(Boolean(''));       //false
console.log(Boolean(' '));      //true

console.log(Boolean(null));       //false
console.log(Boolean(undefined));  //false

自动转换，隐式转换

console.log(" "+ 123);      //"123"
console.log(" 123 " - 1);     // 122    +-*/%,++,--
console.log(true+1);        //2
console.log(" 12 3 " - 1);     // NaN

%,++

    
console.log(12.1 % 2);  //0.09999999999999964
console.log(-12 % 5);   // -2
console.log(12 % (-5)); //  2



let a = 10;
let b;

b = a++;
// b = ++a;

console.log(a); //11
console.log(b); //10

关系运算符 == === > >= < <= != !== 赋值运算符 逻辑运算符 && || & | typeof ?:

console.log('12' != 12);    //false

console.log('12' !== 12);   //true


//如果表达式1为真，那么就直接返回表达式2的值
console.log(10 && 0);   //0
console.log(0 && 12);   //0

console.log(10 || 0);   //10
console.log(!10 || 0);   //0
console.log(0 || 12);   //12
console.log(!0 || 12);   //true



let i = 12;
console.log(12 < 11 && i++);    //false
console.log(i);     //12

let i = 12;
console.log(12 < 11 & i++);    //0
console.log(i);     //13

let i = 12;
console.log(12 < 11 | i++);    //12
console.log(i);     //13

let i = 12;
console.log(12 < 11 | ++i);    //13
console.log(i);     //13



let i = 12;
console.log((i++,++i,i++)); //14

null与undefined

console.log(null == "");
console.log(null == false);
console.log(null == undefined);     //true
console.log(null === undefined);
console.log(null == 0);
//if(a != null)     null与undefined过滤掉了