Start learning simple JS

1. know JS

js Concept ： Run in browser （ client ） The explanation of （ Line by line ） Scripting language .

js engine ： perform js Code ： Execute line by line from top to bottom , Terminate if problems occur .

js The role of ：

Html: structure , skeleton 

css： performance , beautify 

js： Behavior   Dynamic effect （ Shuffling figure .tab Switch . floor . Drag and drop , Baidu search , Form validation ）

js At the heart of ：

<ECMA： standard JS grammar >

<w3c: standard html and css>

1.ECAM-Script: Formulated the js Grammatical norms of

2.BOM: (browser object model) Browser object model , It provides a set of tools to operate the browser API Such as ： Open and close the browser window , Forward go1 back off (go-1)

3.dom:(docuement object madel) Document object model , Provides a set of operation pages （body What's in it API.

 In short ,BOM And DOM It's all through JS  To operate the page .

2.JS Basic grammar of

2.1.js Code writing

1. Inline 

	a Labeled <a href="javascript:js Code ;"> Baidu </a>

	 Not a Labeled ,<button  Behavior properties ="js Code "> Button </button>

	 Events are required to trigger , Such as onclick, Trigger... When clicked .

2. embedded 

	 The way of writing ：<script></script>

	 Writing position ： You can write anywhere JS Code （ It is not recommended to write at the top , Because it is loaded from top to bottom , So it is recommended to write at the bottom , There can be multiple pairs of labels , Load in sequence , No coverage ）

3. Outer chain 

	 The way of writing ：<script src="demo.js"></script>

	 Be careful ：1. In the introduction of the outer chain , Styles within labels are ignored .

				2.demo.js Direct writing js Code .

2.2JS Notes

	 notes ： Multi line comments and single line comments 

	 effect ： Explanation of the code 

	 Single-line comments ： Multiline comment 

2.3js Variable

Variable ： A container for storing data in a program ！

	 How to define variables 

	a) The most common way ：

		var x = 20000; Declare a variable , At the same time x assignment 20000

		var  For declaring variables （ Tell the computer ,x It's a new variable ）

	b) The other way 

		var x; Declare a variable 

		x=20; Assignment after declaration 

		var a,b,c; Declare multiple variables 

		a=20;

		b=20:

		 Declare multiple variables and assign values .

		var a=20,b,v; You can also add an assignment to the declaration 

2.4 Output syntax of variables

		alert（ Content ）

		 effect ： Pop up content 

		console.log( Content )

		 effect ： Output content in the console 

		document.write( Content )

		 effect ： Output in the page 

2.5JS Naming rules and specifications for

		 The rules ：

		1. Part of the ： Numbers 、 Letter 、 Underline 、￥

		2. Do not start with a number 

		3. Can't be keyword （var if switch care for while do) Or reserved words 

		4. Case sensitive 

		 standard 

		1. Hump nomenclature   example ：appplePrice   redApplePrice

		2. See the name and know the meaning ：

		3. Chinese is not recommended 

2.6JS Data type and type conversion

		1. value type =number

		 All arrays are numeric types （ Including binary 、 Hexadecimal 、 octal ）

		NaN  It's not a number 

		- Integers 

		- decimal 

		- Scientific enumeration  10e5

		- Decimal system   octal   Hexadecimal   Binary system 

		-NaN（not a number) Not numbers 

		2. String type =string

		 Everything enclosed in quotation marks is a string 

		-”123“

		-‘123’

		3. Boolean type =boolean

		 Only two. （true and false)

		-true 

		-false

		4.null type = Empty type 

		 There is only one null

notes ！： The detection return value is object

		5.undefined type = Undefined type 

		 only one undefined, No value 

		 Only declare unassigned ：var a;

		- A declared variable can be assigned a value of  undefined

2.7 data type

 The detection shall be carried out in cooperation with the output , Such as ：console.log(typeof('123'))

typeof: Keyword detection

		 grammar ：

			typeof（ The value to detect ）

			typeof The value to detect 

			 Return value （ What happens ）

			1. A certain data type （number string undefined object boolean)

			2. Returns... As a string 

isNaN:（is not number) Check whether a value is a number , If it is , Output is false

			var n1 = 100; console.log(isNaN(n1)); //=> false

			var s1 = 'Jack' console.log(isNaN(s1)); //=> true

2.8 Conversion of data types

			 Conversion between data types , Conversion between various data types 

2.8.1 For example, convert other data types to numerical values ：

			 Method 1 ：

					number（ Variable ）

							 You can cast a variable to a bit value 

							 Can convert decimal places , Keep decimals 

							 Boolean values can be converted 

										 If you encounter something that cannot be converted, you will return NaN

			 Method 2 ：

					parseInt( Variable )

						 Check from the first , If it's a number, change it , Know that the first thing is not a number , Until a content that is not a number . If the beginning is not a number, it will be returned directly NaN.

						 I don't know the decimal point , Only integers can be retained 

			 Method 3 ：

					parseFloat( Variable )

						 Check from the first , If it's a number, change it , Know that the first thing is not a number , It doesn't start with a number , Put it back directly NaN

						 difference ： Know the decimal point once 

2.8.2 Convert other data types to strings

						- Variable .toString()

						-string( Variable )

						- Use + operation 

2.8.3 Other data types are converted to Boolean

						-Boolean（ Variable ） stay js There are only ‘’、0、null、NaN These things false, What is the rest true.

JS The first day continued

Operator

1. Mathematical operators

1.+

	 Only when the symbol is a number twice will the addition operation be performed 

	 As long as either side of the symbol is a string type , String concatenation will be done 

2.-

	 Can perform multiplication 

	 It will automatically convert the values of the two passes into numbers for operation 

3.*

	 Can perform multiplication 

	 It will automatically convert the values of the two passes into numbers for operation 

4./

	 Can do Division 

	 It will automatically convert the values of the two passes into numbers for operation 

5.%

	 Can calculate the remainder 

	 It will automatically convert the values of the two passes into numbers for operation 

2. Assignment operator

1.=

 It's the assignment operation , Assign the value on the right of the equal sign to the variable on the left 

2.+=

var a = 10;

a += 10;

console.log(a);

// The value is 20

a += 10 Equivalent to a= a + 10

3.-=

var a = 10;

a -=10;

console.log(a);// The value is 0

a -=10 Equivalent to a = a - 10;

4.*=

a =10 Equivalent to a = a 10;

5./=

a /=10 Equivalent to a = a / 10;

6.%=

a %=10 Equivalent to a = a % 10;

Comparison operator

1.==

	 Compare the values on both sides to see if they are equal , Regardless of the data type 

	1=='1'  obtain true

2.===

	 Absolute equals sign 

	 Compare whether the values on both sides are exactly equal , Including data types 

	1=='1'  obtain FALSE

3.!=

	 It's not equal to   Compare the values on both sides to see if they are equal , When equal, we get FALSE

	1!='1'  obtain FALSE

4.!==

	 Compare whether the values on both sides are completely unequal , When equal, we get FALSE

	1!=='1' obtain  TRUE

5.>=

	 Compare the values on both sides to see if the left is greater than or equal to the right 

6.<=

	 Compare the values on both sides to see if the left is less than the right 

notes ： Note the writing order of the operators

7.>

8.<

Logical operators

1.&&

 also ： Both left and right sides need to be at the same time true It will take effect. 

2.||

 or ： Only one is needed for the left and right true Effective 

3.！

 Reverse operation , Contrary to its own value 

Auto increment and auto decrement operator

1.++

	 Perform self increment operation 

	 Divided into front ++ And post ++

	var a=10;

	 In front of ++ Such as ,++a, Output is 11;

	 After ++ Such as ,a++, Output is a  Then take it. a The value of the into 11

2.--

	 Perform self subtraction 

	 The usage is the same as self addition