The basic syntax of JavaScript you need to know

1.javascript output

The command issued by the JavaScript statement to the browser. The purpose of a statement is to tell the browser what to do.

<script>
     document.write("hello world!");
</script>

Insert, output.

document.getElementById("pid").innerHTML="jikexueyuan.com";

2. Semicolon

The separation between statements is a semicolon (;)

Note: The semicolon is optional, sometimes See those that are not separated by semicolons.

3. JavaScript code:

Execute in order of writing

4. Identifier:

JavaScript identifiers must begin with a letter, underscore, or dollar sign

JavaScript keywords

5. JavaScript is case-sensitive

JavaScript is also case-sensitive Case is very sensitive.

6. Spaces

JavaScript will ignore extra spaces

7. Code line breaks

Line breaks between words are not allowed

8. Reserved words

##Comments

Single-line comments //

Multi-line comments /**/

JavaScript variables

Variables are "containers" used to store information

var x=10;
var y=10.1;
var z="hello";

<script>
     var i=10;
     var j=10;
     var m=i+j;
     document.write(m);
</script>

JavaScript Data Type

1. String (String)

2. Number (Number)

3. Boolean )

4. Array

The first way to define an array:

var arr=["hello","jike","xueyuan","women"];
document.write(arr[0]);

The second way to define an array:

var arr=new Array("hello","jike","nihao");
document.write(arr[2]);

The third way Method to define numbers, dynamic

var arr=new Array();
arr[0]=10;
arr[1]=20;
arr[2]=30;
document.write(arr[2]);

5.Object(Object)

6.Empty(null)

var n=null;

7.Undefined

var r;

8 .You can clear variables by assigning a value to null

var i=10;
var i=null;
document.write("i");    //null

JavaScript Operator

1. Arithmetic Operator

+ - * % / ++ --

<p>i=10,j=10,i+j=?</p>
<p id="mySum"></p>
<button onclick="jisuan()">结果<button>
<script>
function jisuan(){
     var i=10;
     var j=10;
     var m=i+j;
     document.getElementById("mySum").innerHTML=m;
}
     
</script>

2. Assignment operator

= += -= *= /= %=

3. String operation (string splicing)

Any type added to a string will be converted into a string type

function musum(){
     var i=5;
     var j="5";
     var m=i+j;
     document.getElementById("sumid").innerHTML=m;
}
// 55

4. Comparison operation Symbols

== === != !==  >  <

#5. Logical operators

## && || !

6. Conditional operators

x<10 Compare size

Ternary operation Judgment condition? Output when the condition is met. : Output when the condition is not met

JavaScript conditional statement

if...else..

<script>
     var i=19;
     if(i>=10){
          document.write("我就喜欢i>=10");
     }else{
          document.write("为什么i<10");
     }
</script>

if....else if...else....

<script>
     var i=10;
     if(i>10){
          document.write("我就喜欢i>=10");
     }else if(i<10){
          document.write("为什么i<10?");
     }else{
          document.write("i到底是多少，原来是10");
     }
</script>

switch

swicth(执行条件){
     代码段
}

<script>
     var i=5;
     switch(i){
          case 1:
               document.write("i=1");
               break;  //跳出当前
          case 2:
               document.write("i=2");
               break;
          default:
                document.write("条件不满足case");
                break;  
     }
</script>

Ternary operation

JavaScript loop statement

for loop , for/in

<script>
     var i=[1,2,3,4,5,6];
     for(j=0;j<6;j++){
          document.write(i[j]+"、");
     }
</script>

<script>
     var j=[1,2,3,4,5,6];
     var j=0;
     for(;j<6;){
          document.write(i[j]+"、");
          j++;
     }
</script>

<script>
     var i=[1,2,3,4,5,6];
     var j;
     for(j in i){  //j是i中的元素
          document.write(i[j]+"<br/>");
     }
</script>

while loop, do....while

<script>
     var i=1;
     while(i<10){
          document.write("i="+i+"<br/>");
          i++;
     }
</script>

<script>
     var i=1;
     do{
          document.write("i="+i+"<br/>");
     }while(i<10){
     }
</script>

while is to judge the condition first and then execute it, do.. .while... is executed first and then the condition is judged

JavaScript jump statement

break

Jump out Current cycle

<script>
     for(var i=0;i<10;i++){
          if(i==5){
               break;
          }
          document.write("i="+i+"<br/>");
     }
</script>

continue

End this cycle and continue the next cycle

<script>
     for(var i=0;i<10;i++){
          if(i==5){
               continue;
          }
          document.write("i="+i+"<br/>");
     }
</script>

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