day41 js js basic operations and BOM and DOM

day41 js js basic operations and BOM and DOM

html特殊符号
      空格
    >  >
    &lt;  <

list-style: none;和ul标签的type='none'是一样的

Process Control

if the judge

if (a == 1){  //判断条件写在小括号里面,大括号里面写条件判断成功后的代码内容
    console.log('1111');
}
else{
   console.log('222');
}

Multi-conditional

    var a = 0;
    if(a > 1){
        // console.log('1111');
        // var hhhh = document.getElementById('d1');
        // hhhh.innerText = '彭于晏';
    }else if(a<1){

        console.log('2222');
    }else {
        console.log('3333');
    }

Operators

Comparison operation

> < == !=  >=  <=   ===  !==

var a = 2;
var b = '2';
a == b;  true  弱等于
a === b;  false  强等于
a != b;  false
a !== b;   true

Arithmetic

+  -  * / %   ++  --  
++ 自增 1  
-- 自减 1

var a = 2;
a++  先执行逻辑  在+1
++a  先+1 在执行逻辑

简单示例:
    if (++a === 4){
        console.log('xxx');
    }
    else{
        console.log('ooo');
    };

logic operation

与 &&
或 ||
非 !

switch judgment

If the latter case without break, once the conditions are met, the code behind will all be executed, even if the condition does not hold back.

var num = 200;
switch(num++){
    case 10:
        console.log('未成年');
        break;
    case 18:
        console.log('成年');
        break;
    case 35:
        console.log('油腻老男人');
        break;
    case 100:
        console.log('....');
        break;
    default:
        console.log('太大了');
};

Exception trap

We will catch all exceptions. JavaScript is a programming language difficult to error.

try{
    console.log(xx);
}catch(e){
    console.log(e);
}
finally{
    console.log('sssss');
}

cycle

for loop

for (var i=0;i<100;++i){
    console.log(i);  
};

循环数组
    var d = [11,22,33];
    for (var i in d){
        if (d[i] === 22){
            continue;
            // break;
        }
        console.log(i,d[i]);
    }
    for (var i=0;i<d.length;i++){
        console.log(i,d[i]);
    };
循环自定义对象--python字典
    for (var i in d){
        console.log(i,d[i]);  #不要用d.i来取值
    }

while loop

var a = 0;
while(a<5){
    a++;
    if (a===2){
      continue;
    }
    console.log(a);
}

js basic data types are Boolean []property: 0, {}, '', undefined, null, , NaNrepresent false.

字符串转数字:
    var a = '11';
    parseInt(a);
    
    var a = '23abc';
    parseInt(a);   23
    
    var a = 'asdfabc';
    parseInt(a); // NAN  -- not a number
    typeof NaN;  // "number"
    NaN === NaN; // false
    NaN == NaN; // false

function

Ordinary function

function f1(a,b){
    return a+b;
}
执行: f1(1,2) -- 3
function f1(a,b){
    return a,b;
};

f1(1,2);
// 不能返回多个值，只会返回最后一个值:  2

Anonymous function

var a = function (a,b){
    console.log('xxx');
}

var d = {'xx':'oo','f':function (a,b){
    console.log('xxx');
}};
// 执行:d.f(1,2);

Self-executing function

(function () {
    alert('自执行函数!')
})()

Serialization

var d = {'a':'aa','b':18};
序列化:
    var d_json = JSON.stringify(d); //python  json.dumps(d);
反序列化:
    d_json = "{"a":"aa","b":18}"
    var reverse_json = JSON.parse(d_json);

format

var website = 'https://www.baidu.com'
var web_name = '百度'
var tag = `<a href='${website}'>${web_name}</a>`    // 待格式化的字符串用反引号括起来

BOM Object Browser Object Model

Bomb box

alert('xx');
confirm('are you sure?')

The location object

location.href;  // 获取当前页面的地址
location.href = 'http://www.baidu.com'; // 跳转到这个网址上
location.reload();  // 刷新当前页面

Timer

第一种:一段时间之后执行某个任务
    设置:var t = setTimeout(function(){confirm('你满18岁了吗?')},5000);
        var t = setTimeout("console.log('xxx')",1000);
        t就是浏览器用来记录你的计时器的标识数字
    清除:clearTimeout(t)
第二种:每隔一段时间执行某个任务
    设置:var t = setInterval(function(){confirm('弹个框!!')},3000);
    清除:clearInterval(7);

DOM objects

HTML document (.html file)

Find direct selector

html代码:
    <div class="c1" id="d1"></div>
    <div class="c1 c2" id="d2"></div>
css代码:
       .c1{
            background-color: green;
            height: 100px;
            width: 100px;
        }
        .c2{
            background-color: red;
            /*height: 100px;*/
            /*width: 100px;*/
            color:red;
        }

按标签名查找: var divEle = document.getElementsByTagName('div');
按id值查找:  var d1 = document.getElementById('d1');
            示例: d1.style.height = '600px';
按类值查找:var a = document.getElementsByClassName('c1');

Indirect Find selector

var div1 = document.getElementsByClassName('c1')[0]; 
div1.nextElementSibling.style.color = 'red';  // 找下一个兄弟标签,并改了色
div1.previousElementSibling;  // 找上一个兄弟
div1.firstElementChild;  // 找第一个儿子
div1.lastElementChild;  // 找最后一个儿子
div1.children;  // 找所有儿子,是一个数组
div1.parentElement;  // 找到自己的父级标签

Text manipulation

innerText
    获取文本:
        var a = document.getElementById('jd')
        a.innerText;  // 只获取文本内容
    设置文本:
        a.innerText = '<a href="">校花</a>';// 不能识别标签,单纯的文本内容显示
innerHTML
    获取文本    
        var d = document.getElementsByClassName('c1')[0];
        d.innerHTML;  // 获取的内容包含标签
    设置文本:
        d2.innerHTML = '<a href="">校花</a>'; // 能够识别标签,生成标签效果

value of the operation value

input标签
    html:
        <input type="text" name="username" id="username" >
    示例:
        var inp = document.getElementById('username'); // 找到标签
        inp.value;  // 获取值
        inp.value = '200块!';  // 修改值