JavaScript introducing ways
Write code in the Script tag
<Script>
// here to write your JS code
</ script>
Introducing additional JS file
<script src="myscript.js"></script>
JavaScript comment
// a single line comment / * This is a multiline comment * /
Terminator
JavaScript statements to a semicolon (;) at the end
A, JavaScript language foundation
Variable declaration
1. Variable names can be named by numbers, letters, underscores, $ composition , can not start with a number
2. Declare variables (var variable name;) format to be declared
var name = "Alex"; var age = 18;
In ECMA6 latest syntax: You can also use (let variable names;) format statement
let name = 'jason'
var and let the difference between:
var effect is global, the value of a variable modified
let action is a fragmentary, guaranteed value of the variable can not be arbitrarily modified
ES6 add const to declare a constant. Once declared, its value can not be changed
const pi = 3.1415926
Two, JavaScript Data Types
JavaScript has a dynamic type
var x; // this case x is undefined var x =. 1; // this case x is a number var x = " Alex " // x at this time is a string
1. Numerical (Number)
JavaScript does not distinguish between integer and floating-point type, there is only one type of digital
There are a = 12:34 ; There are b = 20 ; var c = 123e5; // 12300000 have d = 123 A-5; // 0.00123
There is also a NaN, represents not a number (Not a Number).
Common methods:
the parseInt ( " 123 " ) // returns 123 the parseInt ( " the ABC " ) // returns NaN, NaN attribute value representing a special non-numeric value. This attribute is used to indicate a numeric value. parseFloat ( " 123.456 " ) // Returns 123.456
the parseInt ( '111jasfjd') // Returns character 111 which both numbers of characters at the beginning of the digital return
the parseInt ('11 .11 ') // Returns 11
2. String
js string splicing is recommended that you use the plus sign (python is not recommended because of low efficiency plus sign)
var a = "Hello" var b = "world'' var c = a + b; console.log(c); // 得到Helloworld
ES6 new syntax template string ( ') identified by anti-quotation marks. That can be used as a normal character string, may be used to define multiple rows of strings, it supports the replacement string.
Use $ {}, a placeholder
// ordinary string `This is plain string! ` @ Multiline text ` This is a multi-line text ` @ string embedded variable var name =" Jason ", Time =" Today "; ` the Hello $ {name}, How are you $ {} `Time?
Common methods:
method | Explanation |
.length | Returns the length |
.trim() | Remove blank |
.trimLeft() | Remove the white space on the left |
.trimRight() | Remove the white space on the right |
.charAt(n) | Returns n characters |
.concat(value, ...) | splice |
.indexOf(substring, start) | Subsequence position |
.substring(from, to) | The acquisition sequence index |
.slice(start, end) | slice |
.toLowerCase() | lower case |
.toUpperCase() | capital |
.split(delimiter, limit) |
3. Boolean value
Different from python, true and false are lowercase
was a = true;
was b = false;
4.null和undefined
Empty string, 0, null, undefined, NaN is false
It represents null value of the variable is empty (null manually empty value of a variable, so that the object becomes variable type, null value), showing only the undefined variables declared, but not yet assigned.
var ss; //undefined
ss=null; //null
5.对象(object)
JavaScript 中的所有事物都是对象:字符串、数值、数组、函数...此外,JavaScript 允许自定义对象(字典)。
JavaScript 提供多个内建对象,比如 String、Date、Array 等等。
对象只是带有属性和方法的特殊数据类型。
数组
数组对象的作用是:使用单独的变量名来存储一系列的值。类似于Python中的列表。
常用方法:
方法 | 说明 |
.length | 数组的大小 |
.push(ele) | 尾部追加元素 |
.pop() | 获取尾部的元素 |
.unshift(ele) | 头部插入元素 |
.shift() | 头部移除元素 |
.slice(start, end) | 切片 |
.reverse() | 反转 |
.join(seq) | 将数组元素连接成字符串 |
.concat(val, ...) | 连接数组 |
.sort() | 排序 |
.forEach() | 将数组的每个元素传递给回调函数 |
.splice() | 删除元素,并向数组添加新元素。 |
.map() | 返回一个数组元素调用函数处理后的值的新数组 |
.forEach() 将数组的每个元素传递给回调函数
语法:
forEach(function(currentValue, index, arr), thisValue)
.splice() 删除元素,并向数组添加新元素。
.map() 返回一个数组元素调用函数处理后的值的新数组
遍历数组中的元素
var a = [10, 20, 30, 40]; for (var i=0;i<a.length;i++) { console.log(i); }
6.类型查询 typeof
typeof "abc" // "string" typeof null // "object" typeof true // "boolean" typeof 123 // "number"
对变量或值调用 typeof 运算符将返回下列值之一:
- undefined - 如果变量是 Undefined 类型的
- boolean - 如果变量是 Boolean 类型的
- number - 如果变量是 Number 类型的
- string - 如果变量是 String 类型的
- object - 如果变量是一种引用类型或 Null 类型的
三、运算符
1.算数运算符
+ - * / % ++ -- var x=10; var res1=x++; var res2=++x; res1; 10 res2; 12 这里由于的x++和++x在出现赋值运算式,x++会先赋值再进行自增1运算,而++x会先进行自增运算再赋值!
2.比较运算符
> >= < <= != == === !==
1 == “1” // true 弱等于 1 === "1" // false 强等于
3.逻辑运算符
&& || !
4.赋值运算符
= += -= *= /=
四、流程控制
1.if-else
var a = 10; if (a > 5){ console.log("yes"); }else { console.log("no"); }
2.if-else if-else
var a = 10; if (a > 5){ console.log("a > 5"); }else if (a < 5) { console.log("a < 5"); }else { console.log("a = 5"); }
3.switch
var day = new Date().getDay(); switch (day) { case 0: console.log("Sunday"); break; case 1: console.log("Monday"); break; default: console.log("...") }
switch中的case子句通常都会加break语句,否则程序会继续执行后续case中的语句。
4.for
for (var i=0;i<10;i++) { console.log(i); }
5.while
var i = 0; while (i < 10) { console.log(i); i++; }
6.三元运算
var a = 1; var b = 2; var c = a > b ? a : b //这里的三元运算顺序是先写判断条件a>b再写条件成立返回的值为a,条件不成立返回的值为b;三元运算可以嵌套使用;
五、函数
函数定义
JavaScript中的函数和Python中的非常类似,只是定义方式有点区别。
// 普通函数定义 function f1() { console.log("Hello world!"); } // 带参数的函数 function f2(a, b) { console.log(arguments); // 内置的arguments对象 console.log(arguments.length); console.log(a, b); } // 带返回值的函数 function sum(a, b){ return a + b; } sum(1, 2); // 调用函数 // 匿名函数方式 var sum = function(a, b){ return a + b; } sum(1, 2); // 立即执行函数 书写立即执行的函数,首先先写两个括号()()这样防止书写混乱 (function(a, b){ return a + b; })(1, 2);
使用‘箭头’(==>)定义函数
var f = v => v; // 等同于 var f = function(v){ return v; }
如果箭头函数不需要参数或需要多个参数,就是用圆括号代表参数部分:
var f = () => 5; // 等同于 var f = function(){return 5}; var sum = (num1, num2) => num1 + num2; // 等同于 var sum = function(num1, num2){ return num1 + num2; //这里的return只能返回一个值,如果想返回多个值需要自己手动给他们包一个数组或对象中 }
六、内置对象和方法
JavaScript中的所有事物都是对象:字符串、数字、数组、日期,等等。在JavaScript中,对象是拥有属性和方法的数据。
我们在学习基本数据类型的时候已经带大家了解了,JavaScript中的Number对象、String对象、Array对象等。
自定义对象
JavaScript的对象(Object)本质上是键值对的集合(Hash结构),但是只能用字符串作为键。
var a = {"name": "Alex", "age": 18}; console.log(a.name); console.log(a["age"]);
遍历对象中的内容:
var a = {"name": "Alex", "age": 18}; for (var i in a){ console.log(i, a[i]); }
创建对象 (使用new关键字)
var person=new Object(); // 创建一个person对象 person.name="Alex"; // person对象的name属性 person.age=18; // person对象的age属性
Date对象 创建Date对象
//方法1:不指定参数 var d1 = new Date(); console.log(d1.toLocaleString()); //方法2:参数为日期字符串 var d2 = new Date("2004/3/20 11:12"); console.log(d2.toLocaleString()); var d3 = new Date("04/03/20 11:12"); console.log(d3.toLocaleString()); //方法3:参数为毫秒数 var d3 = new Date(5000); console.log(d3.toLocaleString()); console.log(d3.toUTCString()); //方法4:参数为年月日小时分钟秒毫秒 var d4 = new Date(2004,2,20,11,12,0,300); console.log(d4.toLocaleString()); //毫秒并不直接显示
Date对象的方法:
var d = new Date(); //getDate() 获取日 //getDay () 获取星期 //getMonth () 获取月(0-11) //getFullYear () 获取完整年份 //getYear () 获取年 //getHours () 获取小时 //getMinutes () 获取分钟 //getSeconds () 获取秒 //getMilliseconds () 获取毫秒 //getTime () 返回累计毫秒数(从1970/1/1午夜)
Json对象(******)
var str1 = '{"name": "Alex", "age": 18}'; var obj1 = {"name": "Alex", "age": 18}; // JSON字符串转换成对象 var obj = JSON.parse(str1); #loads // 对象转换成JSON字符串 var str = JSON.stringify(obj1); #dumps
RegExp对象