Javascript conditional operator. Conditional statements in Javascript - IF-ELSE construct - Conditions in Javascript - Basics. Other conditionals in JavaScript

... 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.

JavaScript has a conditional construct that affects the execution flow of a program. If (in English if) something is, something is true, then do one thing, otherwise (in English else) - do another.

If statement

Let's take a look at how the if statement works, it is simple and does not require much explanation.

If (condition) (code to execute if condition is true)

It's simple: if the condition is true, then the code in the (...) block is executed.

Var digit \u003d 4; if (digit \u003d\u003d 4) (document.write ("digit is 4.");)

Some strange code can be done:

Var digit \u003d 4; if (true) (document.write ("The condition is true.");)

Else statement

An else statement can be used in conjunction with an if statement. It translates to "otherwise" and specifies an alternate code.

Var digit \u003d 4; if (digit

Note the different spelling of curly braces in this example for the if and else statements. It is not obligatory to write this way, both syntaxes are correct.

The else statement can be followed by a new if statement. This will test multiple conditions.

Var digit \u003d 4; if (digit

JavaScript does not have an elseif statement (in one word) like PHP does.

If you only need to execute one statement, then the block curly braces (...) are unnecessary. In our example, you don't need to write them:

Var digit \u003d 4; if (digit

Lies in JavaScript

The if (condition) statement evaluates and converts the parenthesized condition (expression) to the boolian type (true or false).

Let's reiterate that there is a lie in JavaScript.

• Number 0 (zero).
• Empty line "".
• Boolean false :)
• The value is null.
• The value is undefined.
• The NaN (Not a Number) value.

Everything else is true.

A couple of possible errors:

If ("false") document.write ("This is true.
"); if (false) document.write (" This is true.

");

Here you need to distinguish the string "false" (in quotes), from the value of the boolean type false.

If ("") document.write ("This is true.
"); else document.write (" This is false.
");

Here you need to distinguish the string "" (inside a space), from the empty string "". A space inside a string makes it not empty, but a containing character. For the interpreter, it doesn't matter a letter or a space - a symbol is a symbol.

Other conditionals in JavaScript

• JavaScript switch construction.
• Operator question mark

Conditional Operators

Conditional statements let you skip or execute other statements depending on the value of the specified expression. These operators are the decision points in the program and are sometimes also called branching operators.

If you imagine that the program is a road, and the JavaScript interpreter is a traveler walking along it, then conditional statements can be thought of as intersections, where the program code forks into two or more roads, and at such intersections the interpreter must choose which road to take. ...

If / else statement

The if statement is a basic control statement that allows the JavaScript interpreter to make decisions, or more precisely, execute statements based on conditions. The if statement has two forms. First:

if (expression) statement

In this form, the expression is evaluated first. If the result is true, then the statement is executed. If the expression returns false, then the statement is not executed. For example:

If (username \u003d\u003d null) // If username is null or undefined username \u003d "Alex"; // define it

Note that the parentheses around the conditional expression are a required part of the if statement syntax.

The second form of the if statement introduces the else clause, which is executed when the expression returns false. Its syntax is:

if (expression) statement1 else statement2

This form executes statement1 if the expression returns true, and statement2 if the expression returns false. For example:

If (n \u003d\u003d 1) console.log ("Received 1 new message."); else console.log ("Received" + n + "new messages.");

Else if statement

The if / else operator evaluates the value of the expression and executes one or another piece of program code, depending on the result. But what if you need to execute one of many fragments? A possible way to do this is to use an else if statement. It is not formally a stand-alone JavaScript operator; this is just a common programming style that uses a repeating if / else statement:

If (n \u003d\u003d 1) (// Execute block 1) else if (n \u003d\u003d 2) (// Execute block 2) else if (n \u003d\u003d 3) (// Execute block 3) else (// If neither one of the previous else statements was not executed, execute block 4)

There is nothing special about this snippet. It is simply a sequence of if statements, where each if statement is part of the else statement of the previous statement.

Switch statement

The if statement creates branching in the program flow, and multi-position branching can be implemented with multiple else if statements. However, this is not always the best solution, especially if all branches depend on the value of the same expression. In this case, it is wasteful to re-evaluate the value of the same expression in multiple if statements.

The switch statement is designed for just such situations. The switch keyword is followed by an expression in parentheses and a block of code in curly braces:

switch (expression) (statements)

However, the complete syntax of a switch statement is more complex than shown here. Various places in the block are marked with the keyword casefollowed by an expression and a colon character.

When a switch statement is executed, it evaluates the value of the expression and then looks for a case label that matches that value (matching is determined with the identity operator \u003d\u003d\u003d). If a label is found, the code block is executed starting at the first statement following the case label. If no case label with matching value is found, execution starts from the first statement following the special label default:... If there is no default: label, the entire switch block is skipped.

The operation of the switch statement is difficult to explain in words, the explanation looks much clearer with an example. The following switch statement is equivalent to the repeated if / else statements shown in the previous example:

Switch (n) (case 1: // Execute if n \u003d\u003d\u003d 1 // Execute block 1 break; // Stop here case 2: // Execute if n \u003d\u003d\u003d 2 // Execute block 2 break; / / Stop here case 3: // Executed if n \u003d\u003d\u003d 3 // Execute block 3 break; // Stop here default: // If everything else doesn't work ... // Execute block 4 break; // Stop here )

Pay attention to the keyword break at the end of each case block. The break statement transfers control to the end of the switch statement and continues execution of the statements that follow. The case statements in a switch statement only specify the starting point of the code being executed, but do not specify any ending points.

If there are no break statements, the switch statement will start executing the block of code with the case label corresponding to the value of the expression and continue executing the statements until it reaches the end of the block. In rare cases, this is useful for writing code that jumps from one case label to the next, but 99% of the time, you should carefully end each case block with a break statement. (When using switch inside a function, you can use a return statement instead of break. Both of these statements terminate the switch statement and prevent jumping to the next case label.)

Below is a more practical example of using a switch statement, it converts a value to a string in a way that depends on the type of the value:

Function convert (x) (switch (typeof x) (// Convert a number to a hexadecimal integer case "number": return x.toString (16); // Return a quoted string case "string": return "" "+ x + "" "; // Any other type is converted in the usual way default: return x.toString ();)) console.log (convert (1067)); // Result "42b"

Note that in the previous two examples, the case keywords were followed by numbers or string literals. This is how the switch statement is most often used in practice, but the ECMAScript standard allows you to specify arbitrary expressions after the case.

The switch statement first evaluates the expression after the switch keyword and then the case expressions in the order in which they appear until a matching value is found. The fact of a match is determined using the \u003d\u003d\u003d identity operator, not using the \u003d\u003d equality operator, so the expressions must match without any type conversion.

Because not all case expressions are evaluated each time a switch statement is executed, you should avoid using case expressions that have side effects, such as function calls and assignments. It is safest to limit your case expressions to constant expressions.

As explained earlier, if none of the case expressions match the switch expression, the switch statement starts executing the statement labeled default :. If there is no default: label, the body of the switch statement is skipped entirely. Note that in the previous examples, the default: label appears at the end of the body of the switch statement after all case labels. This is a logical and usual place for it, but in fact it can be located anywhere within a switch statement.