基础 · Dear Yuoek:

变量
- 变量类型
输出
注释
作用域
数据类型
- 类型转换

变量

var

var a = 1;
var s = "Yuoek";

let

let c = 1;
c = 5;

const

const PI = 3.14159;

区别
var：全局变量，不在 {} 内，可以先使用再声明定义
let: 局部变量，变量可以修改
const：静态变量，赋值后不可以修改

变量类型

数字

var num = 10;

字符串

var str = "String";

布尔

var b = true;

命名

var _name1 = "Name1";
var $name2 = "Name2";

输出

alert()

alert("Hello, Yuoek");

document.write()

document.write("Hello, Yuoek");

console.log()

console.log("Hello, Yuoek");

示例

  <script>
    console.log("Happy Everyday!")
    var num1 = 10;
    var num2 = 20;
    console.log("The Sum is: " + num1 + num2)
    var obj = {
      id: "2557",
      name: "Yuoek",
      age: "25"
    };
    console.log(obj);
    let message = "Do you best";
    console.log(message);
  </script>

innerHTML

<div id="Yu"></div>

<script>document.getElementById("Yu").innerHTML = "JavaScript Tutorial"</script>

注释

单行

// 这是注释

多行

/*
这也是注释
*/

作用域

var num = "这是全局变量";
function life() {
    var num = "这是局部变量";
    document.write(num);
}

let 同名变量在函数外不生效

  <script>
    let x = 10;
    var y = 20;
    function vl() {
      let x = 100;
      var y = 200;
      document.write("x: " + x + "<br>" + "y: " + y);
    }
    vl();
  </script>

修改变量

var a = 10;
var a = 100;

赋值，使用后再声明

a = 50;
document.write("a 是：" + a);
var a;

静态变量

const b;
b = 100;
// 只能赋值一次，下面报错
b = 200;

使用块作用域

{
    const b = "Yuoek";
}
const b = "Sophie";

静态数组

const arr = ["Javascript", "c", "python"];
const[0] = "haskell";
arr.push("html");
document.write(arr);

静态对象

const cobj{
    name: "Yuoek";
    age: "18";
};
cobj.name = "Sophie";
cobj.age = "17";
document.write(JSON.stringify(cobj));

不先使用再声明

document.write(x);
const x = 10;

Comparison basis	var	let	const
Scope	Function	Block	Block
Hoisted	Yes	No	No
Reassign	Yes	Yes	No
Re-declare	Yes	No	No
Bind This	Yes	No	No

数据类型

string

var s1 = "这是字符串1";
var s2 = '这是字符串2';
var s3 = `这是字符串3`;

numbers

var n1 = 12e8;
var n2 = 20e-8;

boolean

var b = true;
var c = false;

null

var ty;
var name = "Name";
var name = null;

undefined

var ty;
var name = "Name";
var name = undefined;

BigInt

let BigNum = 1535837495935037423925305n;

Symbol

let syb1 = Symbol("520");
let syb2 = Symbol("520");
let same == syb1 == syb2

Object

let name = "Yuoek";
const obj = {
    age: 18;
    name: name;
};

Array

const arr = ["c", "cpp", "python", "java", "html", "css", "javascript"];

Date

let date = new date();

动态类型

let d = 'Yuoe';
d = 18;
d = true;

typeof

let name = "yuoek";
let age = 18;
document.write(typeof name + "<br>")
document.write(typeof age + "<br>")

类型转换

转字符串

document.write("10" + 20 + "<br>");
document.write("10" + true + "<br>");
document.write("10" + null + "<br>");
document.write("10" + undefined + "<br>");

// String()
document.write(typeof String(10))

// toString()
const num = 100;
num.toString()

转数字

document.write(("10" + 20) + "<br>");
document.write(("10" + true) + "<br>");
document.write(("10" + null) + "<br>");
document.write(("10" + undefined) + "<br>");

// num
document.write(Number('100'));
document.write(Number(null));
document.write(Number(false));

转布尔

num = !!0;
num = !! 1;
str = !!"";
str = !!"Hello";


document.write(Boolean("100"));
document.write(Boolean(null));
document.write(Boolean(undefined));

date 转字符串和数字

// to Number
Number(date);
// or
date.getTime();

// to String
String(date);
// or 
date.toString();

Value String conversion Number conversion Boolean conversion
0 “0” 0 false
1 “1” 1 true
“1” “1” 1 true
“0” “0” 0 true
"" "" 0 false
“Hello” “Hello” NaN true
true “true” 1 true
false “false” 0 false
null “null” 0 false
undefined “undefined” NaN false
[50] “50” 50 true
[50, 100] “[50, 100]” NaN true