[JavaScript知识库]JS类型转换

类型转换

1.String->Number

const string = "99";
// 隐式转换
console.log(string * 1 + 77);
// 显式转换
console.log(Number(string) + 77);

2.Number->String

const number = 66;
console.log(typeof number);
// 隐式转换
console.log(typeof (number + ""));
// 显式转换
console.log(typeof String(number));

3.混合String->Number

把位于开头的数值字符串转为数值型

const string = "66.66alison";
console.log(parseInt(string));
console.log(parseFloat(string));

4.String->Array

const name = "alison";
console.log(name.split(""));

5.Array->String

const array = ["yooo", "alison", "ruby", "andy"];
console.log(array.join(""));
console.log(array.join("&"));

6.Array->Number

空数组为0，x单元素数组为x，多元素数组为NaN

console.log(Number([]));
console.log(Number([3]));
console.log(Number([1, 2, 3]));

7.toString()

主要用于将Array（由逗号隔开）、Boolean、Date、Number等对象转换成String。

const array = ["yooo", "alison", "ruby", "andy"];
console.log(array.toString());
const number = 99;
console.log(typeof number.toString());

8.Boolean隐式转换

比较和运算是两件事

（1）比较

在将boolean与number、string比较时，会隐式地将boolean中的true转为1、false转为0

let number = 99;
console.log(number == true);

let hd = '0';
let hd2 = "1";
let hd3 = "99";
console.log(hd == false);
console.log(hd2 == true);
console.log(hd3 == true);

（2）运算

在运算中string和number会被隐式转换为boolean

let number = 99;
if (number) console.log("number");

（3）其他类型转换为Boolean

console.log(Boolean([]));
console.log(Boolean({}));

9.Boolean显式转换

（1）!!

!先将number转换为boolean类型，然后再取反

!!相当于把数值转为布尔

（2）构造函数 Boolean()

// 数值
let number = 0;
number = !!number;
console.log(Boolean(number));

// 字符串
let string = "Alison";
console.log(!!string);
console.log(Boolean(string));

// 数组
let array = [];
console.log(!!array);
console.log(Boolean(array));

// 对象
let object = {};
console.log(!!object);
console.log(Boolean(object));

// 日期
let date = new Date();
console.log(!!date);
console.log(Boolean(date));