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Three JS loop keyword examples (for and while) _ Basics

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Author: User
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Three ways of writing a loop:

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Different types of loops

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


For loop

A For loop is a tool that you will often use when you want to create a loop.

The following is the syntax for the FOR loop:

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


Statement 1 executes before the Loop (code block) starts
Statement 2 defines conditions for running loops (code blocks)
Statement 3 executes after the Loop (code block) has been executed

Instance

Copy Code code as follows:

for (var i=0; i<5; i++)
{
X=x + "The number is" + i + "<br>";
}

Give it a shot yourself.

From the example above, you can see:

Statement 1 Sets the variable (var i=0) before the loop begins.
Statement 2 defines the conditions for a circular operation (I must be less than 5).
Statement 3 Adds a value (i++) after each block of code has been executed.


Statement 1

In general, we use statement 1 to initialize the variable (var i=0) used in the loop.

Statement 1 is optional, that is to say, no statement 1 is used.

You can initialize any (or more) of the values in statement 1:

Instance:

Copy Code code as follows:

for (var i=0,len=cars.length; i<len; i++)
{
document.write (Cars[i] + "<br>");
}

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

Instance:

Copy Code code as follows:

var i=2,len=cars.length;
for (; i<len; i++)
{
document.write (Cars[i] + "<br>");
}

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, and if False is returned, the loop ends.
Tip: If you omit statement 2, you must provide a break within the loop. Otherwise the loop will not stop. This can cause the browser to crash. Read the contents of the break in later chapters of this tutorial.

Statement 3

Typically, statement 3 increases the value of the initial variable.

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

Instance:

Copy Code code as follows:

var i=0,len=cars.length;
for (; i<len;)
{
document.write (Cars[i] + "<br>");
i++;
}

For/in Cycle

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

Instance

Copy Code code as follows:

var person={fname: "John", lname: "Doe", age:25};
for (x in person)
{
Txt=txt + person[x];
}

You'll learn more about the For/in cycle in the chapters on JavaScript objects.

can refer to this article concretely: http://www.jb51.net/w3school/js/js_loop_for.htm

This article is an English version of an article which is originally in the Chinese language on aliyun.com and is provided for information purposes only. This website makes no representation or warranty of any kind, either expressed or implied, as to the accuracy, completeness ownership or reliability of the article or any translations thereof. If you have any concerns or complaints relating to the article, please send an email, providing a detailed description of the concern or complaint, to info-contact@alibabacloud.com. A staff member will contact you within 5 working days. Once verified, infringing content will be removed immediately.

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