Keyword examples of three js loops (for and while)_Basic knowledge-JS Tutorial-php.cn

Keyword examples of three js loops (for and while)_Basic knowledge

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Release： 2016-05-16 15:15:21

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Three ways to write loops:

<!doctype html>
<title>js循环 by 脚本之家</title>
<meta charset="utf-8"/>
<meta name="keywords" content="js循环 by 脚本之家" />
<meta name="description" content="js循环 by 脚本之家" />
</head>
<body>
//while循环
 <script type="text/javascript">
i = 1;
while (i <= 6)
{
document.write("<h" + i+">脚本之家，这是标题"+i);
document.write("</h"+i+">");
i++;
}
</script>
//do_whilel循环
<script type="text/javascript">
i = 1;
do
{
document.write("<h" + i+">jb51.net ,这是标题"+i);
document.write("</h"+i+">");
i++;
}
while(i<=6);
</script>
//for循环 
 <script type="text/javascript">
for(i=1;i<=6;i++)
{
 document.write("<h"+i+">脚本之家,这是标题"+i);
 document.write("</h"+i+">");
}
</script>
</body>
</html>

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Different types of loops

JavaScript supports different types of loops:
•for - loop through a block of code a certain number of times
•for/in - Loop through the properties of an object
•while - Loop through the specified block of code when the specified condition is true
•do/while - also loops through the specified code block when the specified condition is true

For loop

The for loop is a tool you will often use when you want to create a loop.

The following is the syntax of the for loop:

for (statement 1; statement 2; statement 3)
{
The executed code block
}

Statement 1 is executed before the loop (code block) starts
Statement 2 defines the condition for running the loop (block of code)
Statement 3 is executed after the loop (block of code) has been executed

Example

Copy code The code is as follows:

for (var i=0; i<5; i++)

{

x=x + "The number is " + i + "
";

}

Try it for yourself

From the example above, you can see:

Statement 1 sets the variable (var i=0) before the loop starts.
Statement 2 defines the conditions for the loop to run (i must be less than 5).
Statement 3 increments a value (i++) each time the block of code has been executed.

Statement 1

Usually we use statement 1 to initialize the variables used in the loop (var i=0).

Statement 1 is optional, which means you can do it without using statement 1.

You can initialize any (or multiple) values in statement 1:

Example:

Copy code The code is as follows:

for (var i=0,len=cars.length; i
{

document.write(cars[i] + "
");

}

You can also omit statement 1 (for example, when the value has been set before the loop starts):

Example:

Copy code The code is as follows:

var i=2,len=cars.length;

for (; i
{

document.write(cars[i] + "
");

}

Statement 2

Usually statement 2 is used to evaluate the condition of the initial variable.

Statement 2 is also optional.
If statement 2 returns true, the loop starts again, if it returns false, the loop ends.
Tip: If you omit statement 2, you must provide a break inside the loop. Otherwise the cycle cannot be stopped. This may crash the browser. Please read about break later in this tutorial.

Statement 3

Normally statement 3 will increase the value of the initial variable.

Statement 3 is also optional.
Statement 3 can be used in several ways. The increment can be negative (i--), or larger (i=i+15).
Statement 3 can also be omitted (for example, when there is corresponding code inside the loop):

Example:

Copy code The code is as follows:

var i=0,len=cars.length;

for (; i
{

document.write(cars[i] + "
");

i++;

}

For/In loop

JavaScript for/in statement loops through the properties of an object:

Example

Copy code The code is as follows:

var person={fname:"John",lname:"Doe",age:25};

for (x in person)

{

txt=txt + person[x];

}

You'll learn more about for / in loops in the chapter on JavaScript objects.

For details, please refer to this article: http://www.jb51.net/w3school/js/js_loop_for.htm