A basic introduction .JavaScript
A complete JavaScript essentially consists of three parts:
The first part: ECMAScript is the core of
the second part: Document Object Model (DOM) is mainly used to integrate js, css, html
Part II: Browser Object Model (BOM) is mainly used to integrate js and browser
Briefly JavaScript is a lightweight programming language, but he can be inserted into HTML, and then executed by the browser.
2.JavaScript two reference
//第一种：直接在Script标签中引入 如：
<script>
JS代码
</script>
第二种：在Script标签中引入JS文件 如：
<script src="JS博客整理.js"></script>
The basic syntax of 3.JavaScript
1. Comment
/*
* 这里是注释
* */
2. To statement with a semicolon (;) at the end (or not write, but to write the habit)
3. variable declaration
var 变量名 = 变量的值；
Note: The variable can not start with a number, but you can start with $ character, generally not used at the same time, JS is the dynamic type of variables, a variable can be stored in different types of values, languages like C does not have to declare the variable type.
//let命令声明的变量只在局部的代码块中有效
如：for(let i = 0; i<num; i++){
}
该语句中的i只在for循环中有效
//const命令可以声明常量，一旦声明就不可以再改变
如: const PI =3.1415926535;
4.JavaScript data types
1. Value Type
var a =1 var b =3.14 var c =1234e2
2. Special attention NaN, the full name (Not a Number) indicates that this is not a number
//NaN的用法
parsent("1232") //会返回1232
parsent("你真美") //会返回NaN，因为你真美不是一个数
2. String (String)
var String1 = “你”；
var String2 = "好"；
var String3 = String1 + String2；
console.log(String3); //这里即可输出你好二字
//由上面的例子可以看出字符串可以用加号（+）进行连接
3. The common method string
| method | Explanation |
|---|---|
| .length | Returns the length of the string |
| .trim() | Remove the string blank |
| .trimLeft() (.trimRight()) | Removal string left (right) Blank |
| .charAt() | Returns N characters |
| .concat(value,…) | String concatenation |
| .indexOf(substring,start) | Location subsequence |
| .substring(from,to) | The acquisition sequence index |
| .slice(start,end) | slice |
| .toLowerCase() | lower case |
| .toUpperCase() | capital |
Note: To distinguish the difference between substring and slice of
4. Boolean value
var a = true;
var b = false;
Note: the empty string, 0, null, udefined, NaN is false;
5. Detailed null and undefined
- represents null value is empty, empty is typically a variable is null, or empty direct assignment (var a = null).
- undefined means that when you declare a variable but not yet initialized, the variable default value is the undefined.
**
The 5.JavaScript objects (object)
I believe my fellow learned object-oriented programming language, is no stranger to the definition of the object, here a brief introduction to some of the knowledge in JavaScript.
1. Array
定义：var a = {123 , "ABC"}
输出：console.log(a[1]); 输出ABC
2. The array of commonly used method
| method | Explanation |
|---|---|
| .length | Length of the array |
| .push (he) | Additional elements of the tail |
| .pop() | Gets the element tail |
| .unshift(ele) | Head insert elements |
| .shift | Getting element of the head |
| .reverse() | Reverse |
| .join(seq) | The array element is connected to a string |
| .concat(val,…) | Connection array |
| .sort() | Sequence |
| .forEach() | Each element of the array to the callback function |
/*forEach的用法*/
array.forEach（function(v){ 遍历数组元素
console.log(v);
}）
/*splice()的用法*/
splice(index,howmany,item1,...,itemX) 删除数组中的元素
其中index规定从哪里开始删除元素，howmany规定删除多少元素，item1为要添加的元素也可以不写
/*map()的用法*/
array.map(function(v){ 返回该数组经过函数处理后的新数组
处理该数组的语句
})
3. Type the query
用法： typeof "abc" 返回string
typeof null 返回object
typeof true 返回boolean
typeof 123 返回number
Note: typeof is a unary operator, like addition, subtraction, not a statement is not a function
4. Operators
Operators and other languages similar to the description herein only = difference == === No.
1.=是赋值号 如 a = 1;
2.==是若等于 如 1 == "1" 会返回true
3.===是强等于 如 1 === "1" 会返回false
5. Process control
Process control, including if, if else, switch, for, the ternary operator, usage and C language usage when we first learned of similar.
6. function definition
/*函数有三种定义方式*/
1.普通函数的定义
function f1() {
console.log("Hello World");
}
2.带参数函数的定义
function f1(a,b){
console.log(a,b)
}
3.带返回值的函数
function sum(a,b){
retrun a+b;
}
sum(1,2);
4.匿名函数的方式
var sum = function(a,b){
return a+b;
}
sum(1,2);
5.立即执行函数的方式
（function(a,b){
return a+b;
}）（1,2);
7. Global and local variables
Local variables:
the JS variables declared inside a function (var) is a local variable, so it can only be accessed within the function. When the function is finished, the variable will be deleted.
Global variables:
variable declared outside a function are global variables, all functions on the page can access it.
8. lexical analysis (temporarily omitted, still do not understand)
**
6.JavaScript some built-in objects
NOTE: The difference var s1 = "abc" and var s2 = new string ( "abc") is that the former is returned string objects, which returns the object
1. Object types are described in
| Types of | Built-in objects | Introduction |
|---|---|---|
| type of data | Number | Digital Object |
| String | String object | |
| Boolean | Boolean value object | |
| Grouping objects | Array | An array of objects |
| Math | Mathematical objects | |
| Date | Date Object | |
| Advanced Object | Object | Custom object |
| Error | Error object | |
| Function | Function object | |
| RegExp | Regular expression object | |
| Global | Global Objects |
2. The custom object
on JavaScript object (Object) is essentially a set of key-value pairs, but only as a key string.
var a = {"name":"wunan","age":20};
console.log(a.age);
console.log(a["age"])；
二者返回值都为20
To traverse all the contents of the object:
var a = (var i in a){
console.log(i,a[i]);
}
Create Object
var person = new object(); //创建一个person对象
person.name = "wunan"; //给person对象中的name属性赋值wunan
person.age = 20； //给person对象中的age属性赋值20
JSON object
var str1 = '{"name":"wunan","age":20}';
var obj1 = {"name":"wunan","age":20};
//将JSON字符串转换成对象
var obj = JSON.parse(str1);
//将对象转换成JSON字符串
var str = JSON.stringify(obj1);