Javascript if two conditions are met, print a message. Conditional operators. Conditional if statement

... If you entered a different value, print a note and end the program:
") else (alert (" Enter the correct type "); exit;)
• Initialize a variable kolvo the value entered by the user into the modal:

var kolvo \u003d prompt ("Enter the number of items");

• To convert a string value to a numeric value, use the parseInt function:

for (var i \u003d 1; i<=kolvo; i++) document.write("");

• Since the lists are closed with the corresponding tags, depending on the type of the list, print the closing tags:

if (listType \u003d\u003d "1") document.write ("") else if (listType \u003d\u003d" 2 ") document.write ("" ) ;

if (listType \u003d\u003d "1") document.write ("

") else if (listType \u003d\u003d" 2 ") document.write ("

Js task 3_11.
Write a script that displays tags input(controls) of different types, depending on the entered digit:

1 - text field,
2 - button,
3 - radio(switch).

The number of tags displayed should also be requested.

Let's remember the tags:

For 1 - text field: For 2 - button: For 3 - radio:

Output example:

Js task 3_12. Draw a 9x9 checkerboard using javascript for loops. "Draw" the board should be html tags for the table:

Let's remember the tags:

• To draw 9 lines, you need to organize an external for loop with a counter i.
• To draw 9 cells in each line, you need to organize an inner (nested) for loop with a counter j.
• Use the document.write method to render cell and line tags.

Result:

Additionally:

1. Print the multiplication table into the table cells using the loop counters (i and j).
2. Display the first row and the first column with a red background (attribute of the table cell bgcolor):

   <td bgcolor \u003d "red"\u003e-</ td\u003e

   -

Result:

Questions for self-control:

1. Explain what does dynamic page building mean?
2. What is the most commonly used language construct for dynamic page building?

Cyclic statements of the javaScript language - While

The syntax for the while statement is:

while (condition) (//..block of statements ..);

Example: Display powers of two up to 1000 (2, 4, 8 ... 512). Use alert () method

• Listing of the script:

1 2 3 4 5

var a \u003d 1; while (a< 1000 ) { a*= 2 ; alert(a) ; }

var a \u003d 1; while (a< 1000){ a*=2; alert(a); }

a * \u003d 2 → the operation of compound assignment is used: the product combined with the assignment, i.e. same as a \u003d a * 2

• Test the result in a browser.

How do break and continue statements work in a while loop?

Example:

var a \u003d 1; while (a< 1000){ a*=2; if (a==64) continue; if (a==256) break; alert(a); }

Powers of two will be output to 128 inclusive, and the value 64 will be skipped. Those. in the dialog boxes we will see: 2 4 8 16 32 128

Js task 3_13. What values \u200b\u200bwill the following code snippet display?

var counter \u003d 5; while (counter< 10) { counter++; document.write("Counter " + counter); break; document.write("Эта строка не выполнится."); }

Js task 3_14. Write erection code x to the extent yusing a while loop. Query variable values \u200b\u200band output the result using alert ().

Complete the code:

var x \u003d ...; var y \u003d ...; counter \u003d 1; chislo \u003d x; while (...) (chislo \u003d x * ...; counter \u003d ...;) alert (chislo);

A Correct the error in the program designed to find the factorial of a number:

var counter \u003d prompt ("Enter a number"); var factorial \u003d 1; document.write ("Number factorial:" + counter + "! \u003d"); do (if (counter \u003d\u003d 0) (factorial \u003d 1; break;) factorial \u003d factorial / counter; counter \u003d counter + 1;) while (counter\u003e 0); document.write (factorial);

Js task 3_16. Modify the program for user input:

Prompt for a username until the user actually enters a name (i.e. the field is actually filled in and the cancel key is not pressed). When the name is entered, then output "Hello name!"... document.

How to find errors in javascript?

In some cases, the code on the page does not work for some reason. Where to look for the error? In such cases, you can use the try..catch statement.

The try..catch statement tries to execute a piece of code, and if there is an error in the code, it is possible to display an error on the screen.
The error is stored in the e.message object.

Let's consider the operator's work using an example:

Example: write a statement with an error in the program. Check for an error in the suspected erroneous code: if there is an error in the code, display a message "error handling: error name"... After checking the erroneous operator, regardless of whether there is an error in the code, issue the message "finishing actions"

• As a message with an error, we will use the prompt () method, written with an error - promt ()... Enclose the error message in a try block:

alert ("before"); try (promt ("enter a number"); // operator with an error)

Try from English. - "try", thus, we put a try statement in front of a code fragment that may contain an error (in our case, there really is an error).

• The error message should be placed in a catch block:

6 7 8 9

catch (e) (alert ( "error handling:"+ e.message); )

catch (e) (alert ("error handling:" + e.message);)

If there really is an error, then the catch statement stores this error in the e object. Later it can be displayed in the dialog box - e.message.

• Place the final message, which should be displayed regardless of whether there is an error in the code, in a finally block:

finally (alert ("finishing actions");) alert ("after");

If there is an error, then the interpreter after displaying it in our example will go on to execute the catch block, and then finally (from the English "completion", "finally"), which will always be executed, regardless of whether there was an error or not. Even if there was an error in the catch block.

Important: The finally block is optional in the construction.

Js task 3_17. Follow the example above with the following modifications:

Summary:

The lesson covered the following javascript operators and constructions:

Javascript conditional statements:

Loop operators:

Final task Js 3_18.
Create a game for two:

1. The program asks the first player to enter a number from 1 before 100 (the second player does not see the entered number). Then the second player is asked to guess the entered number. The message is displayed in response "few" or "lot" depending on the answer entered. If the player guesses correctly, congratulations are displayed. If he guesses wrong, the game continues (until the number is actually guessed).
2. Calculate the number of attempts and return the result when the number is guessed.

Questions for self-control:

1. When is it advisable to use a For In loop? What is an example of its use.
2. What is the purpose of the try..catch statement?
3. Explain the purpose of each try..catch statement block.

The source for this interactive example is stored in a GitHub repository. If you "d like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

Syntax

Description

Multiple if ... else statements can be nested to create an else if clause. Note that there is no elseif (in one word) keyword in JavaScript.

If (condition1) statement1 else if (condition2) statement2 else if (condition3) statement3 ... else statementN

To see how this works, this is how it would look if the nesting were properly indented:

If (condition1) statement1 else if (condition2) statement2 else if (condition3) ...

To execute multiple statements within a clause, use a block statement ((...)) to group those statements. In general, it is a good practice to always use block statements, especially in code involving nested if statements:

If (condition) (statements1) else (statements2)

Do not confuse the primitive Boolean values \u200b\u200btrue and false with truthiness or falsiness of the Boolean object. Any value that is not false, undefined, , 0, -0, NaN, or the empty string (""), and any object, including a Boolean object whose value is false, is considered truthy when used as the condition. For example:

Var b \u003d new Boolean (false); if (b) // this condition is truthy

Examples

Using if ... else

Using else if

Note that there is no elseif syntax in JavaScript. However, you can write it with a space between else and if:

If (x\u003e 50) (/ * do the right thing * /) else if (x\u003e 5) (/ * do the right thing * /) else (/ * do the right thing * /)

Assignment within the conditional expression

It is advisable to not use simple assignments in a conditional expression, because the assignment can be confused with equality when glancing over the code. For example, do not use the following code:

If (x \u003d y) (/ * do the right thing * /)

If you need to use an assignment in a conditional expression, a common practice is to put additional parentheses around the assignment. For example:

If ((x \u003d y)) (/ * do the right thing * /)

Specifications

Browser compatibility

The compatibility table on this page is generated from structured data. If you "d like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

Update compatibility data on GitHub

In this example, we first declare four variables using the var keyword, and immediately assign numeric values \u200b\u200bto them. Next, using the operators of increment and decrement, we change the values \u200b\u200bof the numbers. Information is displayed using the function Echo (see article ""). In order not to write the name of the object once again, I used the construction with ().

Logical operators

Logical operators are used when checking the condition, so as not to repeat, I will make an abbreviation: the left operand is L.O., and the right operand is P.O.

• && - Logical "AND"
• || - "OR"
• ! - "NOT"
• \u003e - L.O. more P.O.
• \u003e \u003d - L.O. is greater than or equal to P.O.
• < - Л.О. меньше П.О.
• <= - Л.О. меньше или равен П.О.
• \u003d\u003d - L.O. equal to P.O.
• ! \u003d - L.O. is not equal to P.O.
• | \u003d - L.O. equal to itself OR P.O.
• & \u003d - L.O. equal to myself AND P.O.
• ^ \u003d - EXCLUSIVE OR

Now consider the following script:

As in the previous script, here I used the construction with to shorten the program code. However, to display information, we used the function Popup (see article ""). As a result, the dialog boxes will close automatically after a few seconds. Please note that in this example we did not use curly braces. in the js if conditional statement, they are relevant only when you need to execute more than one line of code, but several.

Finally, let's look at a practical example like solving a quadratic equation:

In everyday life, it is often necessary to make some kind of decision, depending on some condition. For example, if the weather is warm on the weekend, then we will go to the sea, otherwise, if it is cloudy, we will sit at home.

In programming, this is also very common. For this there are two conditional statements, these are if-else and switch-case... In this article I will tell you about the if-else operator, and in the next article about the switch-case.

If-else conditional statement syntax following:

If the condition is true, then the code from the if block is executed, otherwise, if the condition is false, then the code from the else block is executed.

For a better understanding, let's take such a simple example, we have a certain amount of money and we want to buy a car, and here such a condition immediately arises, if we have enough money, then we can buy this car, otherwise we cannot.

Var money \u003d 35000; // Let's say we have 35,000 euros // The car we want to buy costs 50,000 euros. And the following condition occurs if (money\u003e 50000) (document.write ("We can buy a car");) else (document.write ("Not enough money to buy a car");)

We save the document, open it in a browser and see that the following message is displayed on the page "Not enough money to buy a car." If we had more than 50,000 euros, then the code from the if block would be executed. If we had exactly 50,000 euros, then we would also not be able to buy a car, because 50,000 is not more than 50,000. In order for the condition in this case to be true, you need to write a greater than or equal sign (\u003e \u003d) ...

Comment! The logical operation equal is written with two equal signs (\u003d\u003d)... There is also a logical operation less than or equal to (

using curly braces

If there is only one operator, then curly braces are optional, if there is more than one operator in the block, then curly braces are required.

The example above will work fine without curly braces, since both blocks contain only one operator.

Any logical operations can be written inside the ifwhether they are simple or complex. You can also use the AND (&&) and OR (||) operators.

Comment! The else block is optional.

For example, if a is equal to b, and c is equal to d, then we display the corresponding message, otherwise if there is no else block, then we simply move on to the next line.

Var a \u003d 4, b \u003d 4, c \u003d 8, d \u003d 8; if ((a \u003d\u003d b) && (c \u003d\u003d d)) document.write ("a equals b AND c equals d"); document.write ("Next line of code");

Statement if - else if - else

After the if block, one or more else if blocks can follow, and at the end there is an else block. This is useful when you need to use more than one condition.

For a better understanding, let's take an example from everyday life. For example, we have a certain number of outlets. If we have only one outlet in the room, then we can connect only one device, if there are two sockets, then we can connect two devices, and if there are more, then we can connect all devices from home to the electrical network.

Now let's move on to programming.

Var socket \u003d 2; // Number of sockets in the house if (socket \u003d\u003d 1) document.write ("

We can only connect one device

We can only connect two devices

For example TV and laptop

We can connect all devices from home to the electrical network

Depending on the value of the socket variable, this or that block of code will fire. As you probably already understood, if socket is equal to 1, then the first block of code will work. If socket is 2, then the second block of code will fire and if socket has any other value (even a negative number) then the third block of code will fire.

Shorthand if else

The abbreviated notation can be used in the case when, depending on a certain condition, a variable can receive one or another value.

For example, if the value of the variable a is greater than the value of the variable b, then in the variable x we \u200b\u200bwrite the following message, "The variable a is greater than the variable b", otherwise we write that "The variable a is less than the variable b".

Var a \u003d 50, b \u003d 100, x; x \u003d (a\u003e b)? "

Variable a more variable b

Variable a smaller variable b

That's all I wanted to tell you about in this article. The if-else conditional operator is used more than in every script, so it is very important to know and understand it. In the next article I will tell you about one more conditional switch-case statement.