1. The index of the string starts from 0, which means that the first character index value is [0], the second is [1], and so on.

You can use quotation marks in strings, the quotation marks in the string should not be the same as the quotation marks in the string

2. String length

//declare a constant
var txt = "Hello World!";
//Determine the length of a string
document.write( txt.length );

3. Methods of string built-in objects

charAt()	Returns the character at the specified index position
charCodeAt()	Returns the Unicode value of the character at the specified index position
concat()	Concatenates two or more strings and returns the concatenated string
fromCharCode()	Convert Unicode to String
indexOf()	Returns the position in the string that retrieves the first occurrence of the specified character

lastIndexOf()	Returns the position in the string that retrieves the last occurrence of the specified character
localeCompare()	Compare two strings in a local specific order
match()	Find a match for one or more regular expressions
replace()	Replace substrings that match a regular expression
search()	Retrieve values ​​that match a regular expression
slice()	Extract a fragment of a string and return the extracted part in a new string
split()	Split a string into an array of substrings
substr()	Extract the specified number of characters from the string from the starting index number
substring()	Extracts characters between two specified index numbers in a string
toLocaleLowerCase()	Converts strings to lowercase according to the host's locale, only a few languages ​​(eg Turkish) have locale-specific case mappings
toLocaleUpperCase()	Converts strings to uppercase according to the host's locale, only a few languages ​​(such as Turkish) have local-specific sizes

4. Writing method of three attributes

The first: use object

//The first
			//custom object, use object
			var student=new Object()
			//Set some properties on the object
			student.stuID="1001"
			student.stuName="Zhang San"
			student.calssName="MOBILE 2103 CLASS"
			
			//set function to object
			student.sayhellow=function(){
				console.log("big plus sign")
			}
			student.sayhellow()

Second: Constructor

//Second: Constructor
		    function teacher(stuid,stuname){
				//this in the constructor represents the current object
				this.stuid=tid
				this.stuname=tname
				this.Eat=function(){
					console.log("Have a meal")
				}
			}

The third: other methods

//The third method: other methods
			var stu={
				stuid:"1002",
				stuname:"Li Si",
				study:function(){
					console.log("study")
				}
			}
			stu.study()
			console.log(stu.stuid)

5. Example 1:

first number:<input type="text" id="one" /><br />
		second number:<input type="text" id="two"  /><br />
		Operation method:<input type="button" name="" id="jiafa" value="+"  onclick="cal_1('+')"/>
		        <input type="button" name="" id="jianfa" value="-"  onclick="cal_1('-')"/>
				<input type="button" name="" id="chengfa" value="*"  onclick="cal_1('*')"/>
				<input type="button" name="" id="chufa" value="/"  onclick="cal_1('/')"/><br />
				operator:<input type="text" name="result" id="result" value="" />
				<script>
					function cal_1(y){
						//Get the value entered in the textbox
						var one=document.getElementById("one").value
						var two=document.getElementById("two").value
						var result
						if(y=="+"){
							result=parseFloat(one)+parseFloat(two)
						}else if(y=="-"){
							result=parseFloat(one)-parseFloat(two)
						}else if(y=="*"){
							result=parseFloat(one)*parseFloat(two)
						}else{
							result=parseFloat(one)/parseFloat(two)
						}
						document.getElementById("result").value=result
					}
				</script>

Case 2:

first number:<input type="text" id="one" value="" />+
		second number:<input type="text" id="two" value="" />=
		<input type="text" id="result" value="" />
		<input type="button" name="" id="" value="operation"  onclick="cal()"/>
		<script type="text/javascript">
			function cal() {
				var one=document.getElementById("one").value
				var two=document.getElementById("two").value
				var result=parseFloat(one)+parseFloat(two)
				document.getElementById("result").value=result
			}
		</script>