JavaScript Tutorial--Flow Control Statement

The following will bring you an article on the usage of basic JavaScript flow control statements. The editor thinks it’s pretty good, so I’ll share it with you now and give it as a reference. Let’s follow the editor and take a look.

Part 3: Process control statements

The JavaScript code is the writing position:

JavaScript code should be written in the pair of tags 4ec11beb6c39d0703d1751d203c170532cacc6d41bbb37262a98f745aa00fbf0.

Or as an external reference9e49d897a47ad22b2aa56a7aa6704ef72cacc6d41bbb37262a98f745aa00fbf0

Please end each sentence of JavaScript code with a semicolon.

Output statement

1. Console output: console.log();

It can be output in the console Some information, the output information is the content of the parentheses in console.log().

This statement is often used when debugging a program.

2. Pop-up box output:

alert();

Use alert to pop up a prompt box on the web page to display the circle of alert() Information in brackets.

prompt();

Use prompt to pop up an input box on the web page, and use the information in parentheses of prompt() as the prompt information.

confirm();

Contains a pop-up box for confirmation and cancellation.

3. Page output: document.write();

will directly display the content on the page.

Select statement

if....else statement

if (judgment condition/boolean value){
//The following code will be executed if the condition is met

Code 1;

｝else {
/ /When the above conditions are not met, or the boolean value is false, the following code 2 will be executed

Code 2

}

In addition , you can continue to add if judgment after else

if (judgment condition/boolean value){
//If the condition is met, the following code will be executed

Code 1;

｝else if (judgment condition) {
//When the above conditions are not met, or the boolean value is false, the following code 2 will be executed

Code 2

}

else if (judgment condition)

. . . . .

else {
last code

}

Switch ...case statement

Used to determine multiple possible values ​​

switch The statement is most closely related to the if statement, and is also a type of flow control commonly used in other languages. statement.

switch (expression) {
case value: statement
break;
case value: statement
break;
case value: statement
break;
case value: statement
break;

default: statement
}

switch The meaning of each case in the statement is: "If the expression is equal to this value (value), execute The following statement (statement)". The break keyword will cause the code execution flow to jump out of the switch statement. If the break keyword is omitted, the execution of the next case will continue after the current case is completed. . By adding a break statement after each case , you can avoid executing multiple case at the same time. code situation.

It can also be mixed in various situations.

switch (i) {
case 25:
/* 合并两种情形 */
case 35:
alert("25 or 35");
break;
case 45:
alert("45");
break;
default:
alert("Other");
}

It should be noted that the switch statement uses the equality operator when comparing values, so no type conversion will occur (for example,
The string "10" is not equal to the numerical value 10).

break and continue statements

break ends the qualified loop inside the loop.

continue ends this loop inside the loop and starts the next loop:

Loop statement

forLoop is to execute the same piece of code repeatedly.

for(var i = 1; Judgment condition; i++){
Code block to be looped:

}

当程序运行到for的时候，会先声明一个变量i，并且赋值为1，判断i是否满足后面的判断条件，如果满足，执行下面的要循环的代码块，代码 块执行完成之后再执行i++,再判断判断条件是否满足，如果满足再次按照上面的流程执行，如果不满足，结束for循环。

for循环还可以用于嵌套，实现复杂的运算，冒泡排序就用到了for循环的嵌套。下面举两个for循环嵌套的例子。

打印直角三角形，

for (var i = 1; i <= 10; i++) {

for (var j = 1; j <= i; j++){

document.write("☆");

}

document.write("<br/>");

}

打印99乘法表

for (var i = 1; i < 10; i++) {

for (var j = 1; j <= i; j++) {

document.write(j + "*" + i + "=" + i * j);// 1 * 1 = 1

document.write(" ");

}

document.write("<br/>");

}

for in循环

for-in 用于遍历数组或者对象的属性（对数组或者对象的属性进行循环操作）。

举个例子

var x

var mycars = new Array()

mycars[0] = "Saab"

mycars[1] = "Volvo"

mycars[2] = "BMW"

for (x in mycars)

{

document.write(mycars[x] + "<br />")

}

while循环

while(判断条件/boolean){
代码块

}

当代码执行到while的时候，会先判断判断条件是否为true，如果为true，那么会执行while大括号中的代码块，代码块执行完毕以后，再次回到while中再进行判断，如果为true,再次执行while大括号中的代码块，并且再次回while，如果为false就不执行。

注意：将来在写代码的时候一定要注意循环的判断条件不能一直为true,否则会成为一个死循环。

do...while循环

do-while 语句是一种后测试循环语句，即只有在循环体中的代码执行之后，才会测试出口条件。
换句话说，在对条件表达式求值之前，循环体内的代码至少会被执行一次。

do {
statement
} while (expression);

do...while语句在开发中并不怎么用，用的最多的还是for循环，以及for循环嵌套。

补充：

Function函数对象

函数申明式

function fn(){

// 函数体

}

函数表达式(匿名函数)

var fn = function(){

// 函数体

}

// fn表示函数名称

// 函数表达式通常又叫匿名函数 因为没有函数名

函数的调用

fn();// 注意:函数只申明不调用是不会执行的

函数的参数

// 申明

function 函数名(形参1,形参2,形参3,){

// 函数体

}

// 调用

函数名(实参1,实参2,实参3);

函数名(实参1);// 这样写也不会有问题

函数名(实参1,实参2,实参3,实参4);// 这样写也不会有问题

// 注意:函数的实参个数可以和形参的个数不同

函数的返回值

function 函数名(){

return 要返回值;

}

// 注意:函数不写返回值时默认的返回值是undefined

函数的递归：函数在内部调用自身函数叫递归

function fn(){
fn();
}

fn();

函数的回调：被当做参数传递的函数叫回调函数

function fn1(){
console.log(“我是回调函数”);
}

function fn2(parameter){
parameter(); // 调用函数
// 这里的parameter是形参 代表传进来的函数fn1
}

fn2(fn1);// fn1就是一个回调函数

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