What are the program structures in javascript

The program structure in JavaScript includes: 1. Sequential structure, which is executed sentence by sentence from beginning to end; 2. Branch structure, after reaching a certain node, it will decide which one to follow based on the result of a judgment. Branch direction execution; 3. Loop structure.

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer

JavaScript program structure

The execution sequence of the program is divided into three types: sequential structure, branch structure and loop structure

1. Sequential structure

The default structure of the program is executed sentence by sentence from beginning to end

2. Branch structure (select structure)

To After a certain node, it will be decided based on the result of a judgment which branch direction to execute in the future

The characteristics of the branch structure: only one branch will be executed in the same branch structure

(1) if

if(条件){
代码块1；
}

Execution rules: If the condition is established, the code block corresponding to the if statement

var age = 26;
    if (age >= 18) {
   		alert("你已经成年");//你已经成年
    }

is executed in the same Only one branch will be executed in the branch structure. Because the two if conditions are independent, they will output

var a = 5;
       if (a > 3) {
           console.log(1);//1
       }
       if (a > 0) {
           console.log(2);//2
       }

2.if…else

if(条件){
	代码块1；
}else{
	代码块2;
}

Execution rules: if If the condition is true, the code block corresponding to the if statement is executed. If it is not true, the code block in else is executed.

var age = 15;
    if (age >= 18) {
   		alert("你已经成年");
   }else{
   		alert("你还没有成年");//你还没有成年
    }

3. Multi-branch statement

if(条件1){
	代码块1；
}else if(条件2){
	代码块2;
}
...
else if(条件n){
	代码块n;
}else{
	代码块m;
}

Execution rules : When condition n is met and the code block corresponding to condition n is executed, only one branch will be executed

var age = prompt('请输入年龄：');
        if (age < 18) {
            console.log(&#39;未成年&#39;);
        } else if (age >= 18 && age <= 30) {
            console.log(&#39;青年&#39;);
        } else if (age > 30 && age <= 60) {
            console.log(&#39;中年&#39;);
        } else if (age > 60) {
            console.log(&#39;老人&#39;)
        } else {
            console.log(&#39;请输入正确的年龄&#39;);
        }

[Related recommendations: javascript learning tutorial]

4.switch structure

switch...case is a congruent comparison

switch(表达式){
       case 值:
			代码块;
 			break;
		case 值2:
			代码块;
			break;
		...
		default:
			代码块;
			break
	}

Execution rules: The expression is compared with the value behind the case to determine whether the two Equal, if equal, the corresponding code block is executed. If the above case and expression are not equal, the content in default will be executed.

<script>
        var a = 10;
        var b = 20;
        var c = &#39;/&#39;;
        var result;
        switch (c) {
            case "+":
                result = a + b;
                break;
            case "-":
                result = a - b;
                break;
            case "*":
                result = a * b;
                break;
            case "/":
                result = a / b;
                break;
            default:
                result = a + b;
                break;
        }
        console.log(result);
    </script>

Switch penetration problem

 60分以上的及格，其他的留级        
 switch (score) {
            case 6:
            case 7:
            case 8:
            case 9:
            case 10:
                console.log('及格');
                break;
            default:
                console.log('留级');
                break;
        }

3. Loop structure

The loop structure has a loop body, and the loop body is a piece of code. For loop structures, the key is to decide how many times to execute the loop body based on the judgment result;

##1.for

for(循环变量初始化;循环判断;循环迭代){
	循环体;
}

Execution rules

• Step 1: Loop variable initialization var i=0;

• Step 2 : Loop condition judgment i The judgment is established Execute the loop body
  The judgment is not established End the loop

• The third step: Loop iteration i

• Step 4: Return to step 2

  for (var i = 0; i < 5; i++) {
            console.log(i);//0 1 2 3 4
        }

2.while

Execution rules: If If the condition is true, the loop body

while(条件){
		循环体;
	}

is executed to calculate how many times a piece of paper is folded, and the thickness exceeds Mount Everest

var total = 8848000;
        var h = 1;//纸厚度
        var count=0;
        while (h <= total) {            
            h*=2;
            count++;//次数递增
        }
        console.log(&#39;折叠了&#39;+count+&#39;次数&#39;);
        console.log(h);

3.do…while The difference between while and do...whiel: while loop will be judged first and then executed, do...while will be executed first and then judged. Regardless of whether the condition is true or false, it will be executed once

do{
	循环体;
 }while(条件);

 var a = 0;
        do {
            console.log(1);//1
        } while (a > 0);


        while (a > 0) {
            console.log(1);//无输出
            a++;
        }

4.for …in

Commonly used to traverse objects and arrays

数组
var arr = [10, 20, 30, 40];
for(var i in arr){
         console.log(i);
         console.log(arr[i]);
      }
对象
 var obj = {
            name: 'jack',
            age: '20',
            addr: "北京路"
        };
        for (var i in obj) {
            console.log(i);
            console.log(i,obj[i]);
        }

5. The difference between break and continue

break; End the loop and leave yourself The nearest loop

continue; End this loop and continue the next loop, the loop closest to you

 for(var i=0;i<5;i++){
            if(i==2){
                break;
            }
            console.log(i);
        }
        var sum = 0;//需要有初值，否则会NaN
            if (i % 2 != 0) {
                continue;
            }
            sum += i;// sum =sum+0
        }

6. Nesting of loops

The outer loop is executed once, and the inner loop is executed once

 for (var j = 0; j < 4; j++) {
            //输出一行*
            for (var i = 0; i < 5; i++) {
                document.write(&#39;*&#39;);
            }
            //换行
            document.write("<br>");
        };

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