JavaScript Crash Note

什么是js

• 编程语言，在网页上实现功能、交互
• 解释性语言
• 运行在客户端
• 面向对象编程

添加js

内部的js （推荐）

<script>
    // Your JavaScript
<script>

外部的js （推荐）

<script src="script.js"></script>

内联js （不推荐）

<button
onclick="createParagraph()">
Click me
</button>

数据类型

数字

只用浮点型

console.log(2);

console.log(2.5);

console.log(2 + 2);

console.log(0.2 + 0.3);

字符串

console.log("hello");

console.log("hello" + " world");

console.log("hello".length);

console.log("hello".charAt(0));

console.log("hello, world!".replace("hello", "goodbye"));

console.log("hello, world!".toUpperCase());

布尔

console.log(1==1);

console.log(1>2);

console.log(1>=2);

console.log(true && false);

console.log(true || false);

console.log(!true);

Object

var obj = new Object():
var obj1 = {};

obj = {
        name: "MinuteSheep",
        age: "20",
        email: "minutesheep@163.com",
        contact: {
                phone: "1234567",
                telegram: "@minutesheep"
        }
}
console.log(obj);

console.log(obj.name);
console.log(obj.contact.phone);

console.log(obj["name"]);
console.log(obj["contact"]["phone"]);

obj.age = 35;
console.log(obj);

obj.contact.wechat = "12345678";
console.log(obj);

Array

var a = new Array();
var a1 = [];

a[0] = "dog";
a[1] = "cat";
console.log(a);
console.log(a.length);

a[5] = "tiger";
console.log(a.length);      //长度为6
console.log(a[3]);          //输出为未定义

a.push("sheep");            //末尾加
console.log(a);
a.unshift("lion");          //开头加
console.log(a);

a.pop();                    //末尾删
console.log(a);
a.shift();                  //开头删
console.log(a);

a.reverse();
console.log(a);

a1 = ["dog", "cat", "tiger"];
console.log(b);

变量

var name="MinuteSheep";     // var没有作用域，即使定义在局部，依旧可以在全局访问
console.log(name);

let a = 1;                  // let拥有作用域，定义在局部，只能在局部访问
console.log(a);

name = "MinuteWolf";
console.log(name);

a = 1+1;
console.log(a);

const PI = 3.14;    //不可修改
console.log(PI);

运算符

var x;
x = "3" + 4 + 5;    //相当于 x = "3" + "4" + "5";
console.log(x);     //输出345

x = 3 + 4 + "5";
console.log(x);     //输出75

console.log(123 == "123");      //true, 类型自动转换
console.log(123 === "123");     //false, 不转换类型

结构语句

选择语句

var name = "minutesheep";
var age = 20;

if (name == "minutesheep") {
        console.log("true");
} else {
        console.log("false");
}

if (name == "minutesheep" && age == 20) {
        console.log("true");
} else {
        console.log("false");
}


if (name == "minutesheep" && age == 20) {
        console.log("true1");
} else if(name == "john" || age == 35) {
        console.log("true2");
} else {
        console.log("stranger");
}

var allowed = (age >= 18) ? "Yes" : "No";
console.log(allowed);

switch (name) {
        case "minutesheep":
                console.log("minutesheep");
                break;
        case "john":
                console.log("john");
                break;
        default:
                console.log("stranger");
}

循环语句

var name = "minutesheep";
var age = 20;
var animals = ["cat", "dog", "tiger"];

while (true) {
        console.log("true");
}

// do循环先做一次循环
do {
        age++;
        console.log(age);
} while (name == "minutesheep");

for (let i=0; i<5; i++) {           // for循环遇到undefined变量不会跳过
        age++;
        console.log(age);
}

for (let i in animals) {            // for ... in ... 遇到undefined变量会自动跳过
        console.log(animals[i]);
}

函数

let a = 1;

function add(x) {
        a += x;
}

console.log(a);
add(20);
console.log(a);

function sum() {
        let sum = 0;

        for (let i = 0, j = arguments.length; i<j; i++) {
                sum += arguments[i];
        }

        return sum;
}

let sum = sum(1,2,3,4,5,6,7);
console.log(sum);

闭包

function makeAdder(a) {
        return function(b) {
                return a + b;
        };
}

var x = makeAdder(5);
var sum = x(8);
console.log(sum);