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JavaScript - if...else Statement

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JavaScript - if...else Statement

 While writing a program, there may be a situation when you need to adopt one out of a given set of paths. In such cases, you need to use conditional statements that allow your program to make correct decisions and perform right actions.

JavaScript supports conditional statements which are used to perform different actions based on different conditions. Here we will explain the if..else statement.

Flow Chart of if-else

The following flow chart shows how the if-else statement works.

أثناء كتابة برنامج ، قد يكون هناك موقف عندما تحتاج إلى اعتماد واحد من مجموعة مسارات معينة. في مثل هذه الحالات ، تحتاج إلى استخدام الجمل الشرطية التي تسمح لبرنامجك باتخاذ القرارات الصحيحة وتنفيذ الإجراءات الصحيحة. يدعم JavaScript العبارات الشرطية التي تُستخدم لتنفيذ إجراءات مختلفة بناءً على شروط مختلفة. هنا سوف نشرح جملة if .. أخرى. مخطط التدفق لـ if-else يوضح المخطط الانسيابي التالي كيف تعمل عبارة if-else.

Decision Making

JavaScript supports the following forms of if..else statement −

  • if statement

  • if...else statement

  • if...else if... statement.

if statement

The if statement is the fundamental control statement that allows JavaScript to make decisions and execute statements conditionally.

عبارة if هي بيان التحكم الأساسي الذي يسمح لجافا سكريبت باتخاذ القرارات وتنفيذ العبارات بشروط.

Syntax

The syntax for a basic if statement is as follows −

if (expression) {
   Statement(s) to be executed if expression is true
}

Here a JavaScript expression is evaluated. If the resulting value is true, the given statement(s) are executed. If the expression is false, then no statement would be not executed. Most of the times, you will use comparison operators while making decisions.

هنا يتم تقييم تعبير JavaScript. إذا كانت القيمة الناتجة صحيحة ، فسيتم تنفيذ العبارة (العبارات) المحددة. إذا كان التعبير خاطئًا ، فلن يتم تنفيذ أي عبارة. في معظم الأوقات ، ستستخدم عوامل المقارنة أثناء اتخاذ القرارات.

Example

Try the following example to understand how the if statement works.


<html>
   <body>     
      <script type = "text/javascript">
         <!--
            var age = 20;
         
            if( age > 18 ) {
               document.write("<b>Qualifies for driving</b>");
            }
         //-->
      </script>      
      <p>Set the variable to different value and then try...</p>
   </body>
</html>

Output

Qualifies for driving
Set the variable to different value and then try...

if...else statement

The 'if...else' statement is the next form of control statement that allows JavaScript to execute statements in a more controlled way.

Syntax

if (expression) {
   Statement(s) to be executed if expression is true
} else {
   Statement(s) to be executed if expression is false
}

Here JavaScript expression is evaluated. If the resulting value is true, the given statement(s) in the ‘if’ block, are executed. If the expression is false, then the given statement(s) in the else block are executed.

Example

Try the following code to learn how to implement an if-else statement in JavaScript.


<html>
   <body>   
      <script type = "text/javascript">
         <!--
            var age = 15;
         
            if( age > 18 ) {
               document.write("<b>Qualifies for driving</b>");
            } else {
               document.write("<b>Does not qualify for driving</b>");
            }
         //-->
      </script>     
      <p>Set the variable to different value and then try...</p>
   </body>
</html>

Output

Does not qualify for driving
Set the variable to different value and then try...

if...else if... statement

The if...else if... statement is an advanced form of if…else that allows JavaScript to make a correct decision out of several conditions.

Syntax

The syntax of an if-else-if statement is as follows −

if (expression 1) {
   Statement(s) to be executed if expression 1 is true
} else if (expression 2) {
   Statement(s) to be executed if expression 2 is true
} else if (expression 3) {
   Statement(s) to be executed if expression 3 is true
} else {
   Statement(s) to be executed if no expression is true
}

There is nothing special about this code. It is just a series of if statements, where each if is a part of the else clause of the previous statement. Statement(s) are executed based on the true condition, if none of the conditions is true, then the else block is executed.

Example

Try the following code to learn how to implement an if-else-if statement in JavaScript.


<html>
   <body>   
      <script type = "text/javascript">
         <!--
            var book = "maths";
            if( book == "history" ) {
               document.write("<b>History Book</b>");
            } else if( book == "maths" ) {
               document.write("<b>Maths Book</b>");
            } else if( book == "economics" ) {
               document.write("<b>Economics Book</b>");
            } else {
               document.write("<b>Unknown Book</b>");
            }
         //-->
      </script>      
      <p>Set the variable to different value and then try...</p>
   </body>
<html>

Output

Maths Book
Set the variable to different value and then try...
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أخر المواضيع من قسم : Javascript

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