Force exit from loop in javascript. For Loops in JavaScript
A loop is a control instruction that allows you to repeat the execution of program code a certain number of times. Each individual execution of instructions in the body of a loop is called an iteration.
while loop
While loop syntax:
The expression in parentheses is called condition for fulfillment cycle or briefly condition. First, the value of the expression is calculated. The resulting value is implicitly converted to a Boolean type if necessary. If the result of evaluating the expression is true , then the instruction located in the body of the loop is executed, then control is transferred to the beginning of the loop and the condition is evaluated again. If the result of evaluating the expression is false , the interpreter terminates the loop and proceeds to execute the statement following the loop. Thus, the interpreter executes the code located in the body of the loop over and over again as long as the condition remains true:
Var i = 0; while (i< 3) { // Выполнять код, пока значение переменной i меньше 3 alert("i: " + i); i++; // Увеличиваем значение переменной i }
do-while loop
Do-while loop syntax:
A do-while loop is similar to a while loop, except that the loop condition is checked after the first iteration rather than before, and the loop ends with a semicolon. Since the condition is checked after the iteration, the code in the body of the do-while loop is always executed at least once:
Var count = 0; do ( document.write(count + " "); count++; ) while(count< 5); Попробовать »
This loop can be useful when the code in the loop body must be executed at least once, regardless of the execution condition.
for loop
For loop syntax:
A for loop contains three expressions separated by semicolons. These three expressions have the following execution order:
- The first expression is always evaluated only once - before the first iteration. Therefore, usually the first expression is the definition of a variable that is used in the loop execution condition as a counter.
- The second expression defines the condition for executing the loop. It is calculated before each iteration and determines whether the body of the loop will be executed. If the expression evaluates to true, the code in the loop body is executed. If false is returned, the loop ends and control moves to the next instruction after the loop. If the first time the condition is checked, it turns out to be false, the code in the body of the loop will not be executed even once.
- After each iteration, the third expression is evaluated. Typically it is used to change the value of a variable that is used to test the condition of a loop.
Example for loop:
For (var count = 0; count< 5; count++) document.write(count + " "); Попробовать »
As you can see from the example, the for loop, unlike other loops, allows you to group the code associated with the loop in one place.
Any of the expressions in the for loop can be missing, but the semicolons themselves must be present, otherwise there will be a syntax error. If the second expression is missing, the loop will run forever.
Var i = 0; for(;i< 4; i++) ... var i = 0; for (; i < 4;) ... for (var i = 1; /* нет условия */ ; i++) ... // Это эквивалентно следующему коду for (var i = 1; true; i++) ...
Instead of one expression, you can specify several expressions, separating them with the comma operator.
// will not be executed because the last expression in the condition check is false for (i = 1; i< 4, false; i++) ...
for (var i = 1, j = 5; i <= 5; i++, j--)
document.write(i + " " + j +"
"); Try »
for-in loop
A for-in loop is used to iterate through the enumerated properties of an object in random order and has the following syntax:
For (variable in object) statement;
To the left of the in keyword is the name of a variable, which is assigned as a string the name of one of the object's properties before the start of each iteration of the loop. To the right of the in keyword is an object whose properties will be iterated through the loop. The loop will run until all available properties have been iterated. If the variable representing the object is null or undefined, the loop will not execute even once:
Var obj = (x: 5, y: 10); for (var prop in obj) ( alert(prop); )
To keep the code associated with a loop in one place, you can declare a variable in a for-in loop. Therefore, the expression before the in keyword is usually a declaration of a variable to which property names will be assigned.
JavaScript Loops provide repeated execution of repetitive calculations. They optimize the coding process by executing the same statement or block of statements that form the body of a loop a specified number of times (using a counter variable) or while a specified condition is true. Loops iterate over a sequence of values. Executing a loop once is called iteration.
The performance of a loop is affected by the number of iterations and the number of operations performed in the loop body of each iteration.
The following loop operators exist in JavaScript:
1) for is used when you know in advance how many times you need to do something;
2) for...in is used to traverse the properties of objects;
3) while is used when you don’t know how many times you need to do something;
4) do...while works similarly to the while statement. The difference is that do...while always executes the expression inside the curly braces at least once, even if the condition test returns false .
Types of loops in JavaScript, loop control
1. For loop
The for loop is used to iterate through the elements of arrays or array-like objects such as arguments and HTMLCollection. The condition is checked before each iteration of the loop. If the check is successful, the code inside the loop is executed, otherwise the code inside the loop is not executed and the program continues from the first line immediately following the loop.
The next loop will print the line Hello, JavaScript! Five times.
For (var i = 0; i< 5; i++) {
console.log(i + ": Hello, JavaScript!");
}
Rice. 1. Result of executing a for loop on the console
1.1. How the for loop works
The for loop consists of three different operations:
Step 1. initialization var i = 0; — declaration of a counter variable that will be checked during loop execution. This variable is initialized with the value 0. Most often, variables named i, j and k act as loop counters.
Step 2. condition check i< 5; — условное выражение, если оно возвращает true , тело цикла (инструкция в фигурных скобках) будет выполнено. В данном примере проверка условия идёт до тех пор, пока значение счётчика меньше 5 .
Step 3. final operation i++ - counter increment operation, increases the value of the variable var i by one. Instead of the increment operation, the decrement operation can also be used.
At the end of the loop, the variable var i is stored at 1. The next iteration of the loop is executed for (var i = 1; i< 5; i++) { } . Условное выражение вычисляется снова, чтобы проверить, является ли значение счётчика i всё ещё меньше 5 . Если это так, операторы в теле цикла выполняются ещё раз. Завершающая операция снова увеличивает значение переменной на единицу. Шаги 2 и 3 повторяются до тех пор, пока условие i < 5; возвращает true .
1.2. Printing array values
To print the values of an array using a for loop, you need to use the length property of the array. This will help you determine the number of elements in the array and loop the same number of times.
The script below will display five messages with the names of the colors:
Var flowers = ["Rose", "Lily", "Tulip", "Jasmine", "Orchid"]; for (var i = 0; i< flowers.length; i++){ alert(flowers[i] + " - это цветок."); }
If the value of the length property does not change during the loop, you can store it in a local variable and then use that variable in a conditional expression. This way, you can increase the speed of the loop, since the value of the length property will be retrieved only once during the entire duration of the loop.
Var flowers = ["Rose", "Lily", "Tulip", "Jasmine", "Orchid"], len = flowers.length; for (var i = 0; i
2. Loop for...in
For...in loops are used to traverse the properties of non-array objects. This bypass is also called transfer. When traversing, it is recommended to use the hasOwnProperty() method to filter out properties that were inherited from the prototype.
For example, let's create an object using an object literal.
Var user = ( name: "Alice", age: 25, country: "Russia" ); for (var prop in user) ( console.log(prop + ": " + user); ) 
Rice. 2. Result of executing the for...in loop on the console
Suppose that in a scenario before or after the user object was created, the prototype of the Object object was extended with an additional clone() method.
If (typeof Object.prototype.clone === "undefined") ( Object.prototype.clone = function () (); )
Since the prototype inheritance chain is constantly checked by the interpreter, all objects automatically gain access to the new method.
Rice. 3. Result of repeating the for...in loop on the console To avoid detection of this method while enumerating the properties of the user object, the hasOwnProperty() method is used, which will filter out the properties of the prototype.
Var user = ( name: "Alice", age: 25, country: "Russia" ); if (typeof Object.prototype.clone === "undefined") ( Object.prototype.clone = function () (); ) for (var prop in user) ( if (user.hasOwnProperty(prop)) ( console.log (prop + ": " + user); ) ) 
Rice. 4. The result of listing the properties of an object using the hasOwnProperty() method
3. While loop
The while loop is a loop with a preliminary check of a conditional expression. The statement inside the loop (block of code in curly braces) will be executed if the conditional expression evaluates to true . If the first check returns false , the block of instructions will not be executed even once.
After the loop iteration completes, the conditional expression is again tested for truth and the process is repeated until the expression evaluates to false . In this case, the program will continue from the first line immediately after the loop (if there is one).
This loop will display the multiplication table for the number 3:
Var i = 1; var msg = ""; while (i< 10) {
msg+= i + " x 3 = " + (i * 3) + "
"; i++; ) document.write(msg); 
Rice. 5. Result of executing the while loop
4. Do...while loop
Loop do...while; checks the continuation condition after the loop is executed. Unlike the while loop, in do...while; The body of the loop is executed at least once, since the condition is checked at the end of the loop, and not at the beginning. This loop is used less frequently than while , since in practice a situation where at least one loop execution is required is rare.
Var result = ""; var i = 0; do ( i += 1; result += i + " "; ) while (i< 5);
document.write(result);
Rice. 6. Result of executing the do...while loop
In the following example, the statements within a loop are executed once, even if the condition is not true.
Var i = 10; do ( document.write(i + " "); i++; ) while (i< 10);
5. Infinite loops
When you create any loop, you can create an infinite loop that will never end. Such a loop could potentially continue to run as long as the user's computer is running. Most modern browsers can detect this and prompt the user to stop running the script. To avoid creating an infinite loop, you must be sure that the given condition will return false at some point. For example, the following loop specifies a condition that never returns false because i will never be less than 10:
For (var i = 25; i > 10; i++) ( document.write("This sentence will run forever...
");
}
6. Nested loops
A loop inside another loop is called nested. With each iteration of the loop, the nested loop is executed completely. Nested loops can be created using a for loop and a while loop.
For (var count = 1; count< 3; count++) {
document.write(count + ". Строка цикла
"); for (var nestcount = 1; nestcount< 3; nestcount++) {
document.write("Строка вложенного цикла
");
}
}
Rice. 7. Result of executing a nested for loop
7. Cycle management
The loop can be controlled using break statements; and continue; .
7.1. Operator break;
Operator break; ends execution of the current loop. It is used in exceptional cases when the loop cannot execute for some reason, such as if the application encounters an error. Most often the break operator; is part of the if construct.
When the statement break; used without a label, it allows you to exit a loop or switch statement. The following example creates a counter whose values should range from 1 to 99, but the break statement breaks the loop after 14 iterations.
For (var i = 1; i< 100; i++) {
if (i == 15) {
break;
}
document.write(i);
document.write("
");
}
Rice. 8. The result of the break operator in the for loop
For nested loops, the break statement; used with a label that terminates the named instruction. A label allows you to exit any block of code. A named statement can be any statement external to a break statement; . The label can be the name of an if statement or the name of a block of statements enclosed in curly braces just to assign a label to that block. Between the keyword break; and the label name does not allow a newline.
Outerloop: for(var i = 0; i< 10; i++) {
innerloop:
for(var j = 0; j < 10; j++) {
if (j >3) break; // Exit the innermost loop if (i == 2) break innerloop; // Same thing if (i == 4) break outerloop; // Exit outer loop document.write("i = " + i + " j = " + j + "
"); ) ) document.write("FINAL i = " + i + " j = " + j + "
");
7.2. Operator continue;
Operator continue; stops the current loop iteration and starts a new iteration. In this case, the while loop returns directly to its condition, and the for loop first evaluates the increment expression and then returns to the condition.
This example will display all even numbers:
Var i; for(i = 1; i<= 10; i++) {
if (i % 2 !== 0) {
continue;
}
document.write("
even number= " + i); ) 
Rice. 9. The result of the continue operator in the for loop
Operator continue; can also be used in nested loops with a label.
Outerloop: for (var i = 0; i "); for (var j = 0; j "); ) ) document.write("All loops completed"+"
");
Rice. 10. The result of the continue operator with a label
The history of the formation of modern programming language syntax is akin to understanding the processes of formation of the Universe. What and how it was in the beginning... But now everything is simple and accessible.
Ultimately, the algorithm is always a sequential chain of commands. Parallelism in programming is a set of somehow combined sequences. sequential or parallel chain of command has never been more practical. Labels, transitions and conditions - everything was enough for any solution. Functional languages have made these ideas irrelevant, but the need to repeat sections of code remains.
Browser: DOM, its language + server
In JavaScript, loops remain, although functional ideas have acquired a special meaning. There may be something left over from "Lisp" and "Prolog", but most likely the area where JavaScript lives led to what is there, but it is doubtful that this is the last solution.
JavaScript runs inside the browser, which receives the page, parses it into the DOM, and runs the first script. All other pages, including those loaded on this one, are the work of the developer manipulating the language, through which the code on the server can be called and the result obtained using the AJAX mechanism.
The browser executes JavaScript code, which can use browser objects, including the one that transmits information to the server and receives a response, which can be HTML markup, styles, and the code itself. The response can be represented by arrays and objects. The point of using loops in JavaScript is lost, there are plenty of opportunities to do without them, and the risk of hanging the browser with an endless sequence of commands is not the best solution.
Loops themselves are present in most JavaScript syntactic constructs; the developer can supplement the standard constructs with his own functions.
JavaScript's Position in Code Space
A modern programmer does not even think that the while, do while, ...) he uses is ultimately a series of processor cycles, a simple sequence of binary operations, interrupted by counter checks, that is, conditions.
There is no loop as such at the machine language level: there is a combination of ordinary commands, conditional operations and transitions. One level up, no matter what tool is used to develop the browser and JavaScript interpreter, there will definitely be loops. Moreover, “pieces of code” will be presented at different times and by different generations of programmers. The floor above is the JavaScript "building". The syntax of which offers modern JavaScript loops.
JS is a wonderful language: practical, modern and full-featured. The syntax of this tool includes all the constructs that have stood the test of time and have become the unshakable foundation of any algorithm. But are cycles really necessary? Progress in programming often asked itself questions of a fundamental nature, but only in some cases did it find a solution.
Objective grounds
A cycle can have only two options: by condition or by counter, but essentially (at the lowest level) any cycle is only by condition. In some languages there is a cycle "for each". In JavaScript, foreach loops are represented by the prop in object construct, but you can use the array.forEach(...) variant.
In any case, there are two options: the programmer who ultimately executes all the programmer’s algorithms, even those writing in interpretive languages, has no other options for repeating the chain of commands: he can execute something again until:
- the counter counts;
- as long as the condition is met.
JavaScript is a typical interpreter. Its peculiarity: it operates inside the browser, uses its objects and allows you to execute algorithms on the client side, both when the page is loaded into the browser and during its operation.
Simple loop one by one
In JavaScript, foreach loops look like applying a function to an array:
The use of such cycles does not pose any difficulties. Formally, there is no cycle as such. There is a sequential function call to the elements of the array.
Loop on a counter
For loops look more familiar in JavaScript:
Here the counter is a variable whose value changes according to a formula and a condition is a sign of the end of the cycle. The formula and condition do not need to include a loop variable. But control over the moment the cycle ends is completely determined by their content.
Conditional loops
JavaScript offers an option with while depending on when the condition needs to be checked. If the body of the loop may not be executed even once, this is one thing; if the body must be executed at least once, this is another:
In the first case, when interpreting the while construct, JavaScript first checks the condition, and if it is true, it executes the loop. In the second case, the loop will be executed first. If, as a result of changing the variables specified in the design condition do while, it will evaluate to false and the loop will stop executing.
Massive combinations of simple algorithms
The main task (component part) of any algorithm is to find, and only then make a decision about what to do next. The most primitive search option is accessing a variable, the result is obtained directly. If there are many variables, or it has many values (an array), then to select a value it is necessary to find something that will determine the further behavior of the script.
Such a simple doctrine made a loop with a counter in JavaScript is a kind of panacea for all problems. Modern computers are fast. There is plenty of time to run scripts in the browser, there is no need to rush. It’s easier than ever to go through something for the sake of something. As a result, on J AvaScript for loops have become very popular.
There seems to be nothing bad about this. But behind this approach, the essence for which this or that algorithm is written is easily lost. Data is not meaningless. Everything for which any program is written has meaning. Excessive use on J avaScript for loops, the developer may not recognize the required entity and not create an adequate algorithm.
Functionality, another reflection of reality
Applying JavaScript loops, examplescode of the same type can be represented by functions - the algorithm will immediately transform, the main body of the script will decrease in size, everything will become readable and understandable.
This is not a radically new solution, but in essence it does not go beyond other language constructs. In particular, J AvaScript loops can be found in the classic split() function:
var cResult = "9,8,7,6,5,4" ;
var aResult = cResult .split ( "," );
There is no loop here, but how else can this be done other than by looking for the "," symbol and using it to separate one number from another.
Abstracting from how this is implemented inside the split() function, you can supplement JavaScript with your own functionality that uses loops, which is more convenient from a usage point of view. It is important that this approach leads to the development of functionality for each task, respectively, but the general will still be with this approach.
These functions allt(), padc(), padl() and padr() are something that JavaScript doesn't have, but sometimes you need to remove spaces from a string or align the length of a string on the left, right, or both sides. The bodies of these functions contain JavaScript loops. Simple, accessible, and the algorithm that uses this will never crash.
Variants of functions for converting numbers from hexadecimal to 10th number system and vice versa, more simply, from one data format to another, are performed here using do while loops. Very compact and efficient language syntax.
Correct cycles - a reflection of reality
JavaScript is no match for other programming languages and does not differ in the variety of versions, and most importantly, it strives not to change the syntax, but to develop and expand it.
The mindset of a programmer using JS is different from the thinking of a PHP programmer (in particular, and other languages in general, except that “Prolog” and its followers are not included in the general mainstream), when the algorithm is not limited to variables, arrays, assignment operators, and cyclic constructs.
If we imagine that there are no cycles, but the problem needs to be solved, then the simplest option (blindfold) is to assume that the program processes data that is a point or a system of points in the information space. What a point is and what a system of points is is a matter of a specific subject area. For a programmer, this thesis means: there is a simple given and there is a collection of simple data. Naturally, a simple datum of one level will be a system for a level below, and a point for a level above.
With this approach, the point's concern is to manifest its essence through its methods. When a point is in a supersystem, then the function of the system is to manifest its essence as a collection of essences of the points included in it.
This approach is as old as the idea of programming languages, but has not yet been adequately reflected in programming. Many programmers think correctly, but the result of their creativity leaves much to be desired.
It helps to wear a blindfold sometimes to see the world!
Cycles
To understand the effect of conditional statements, we suggested imagining them as forks in the road along which the JavaScript interpreter moves. Loops can be thought of as a U-turn in the road that takes you back, forcing the interpreter to go through the same piece of code over and over again.
JavaScript has four loops: while, do/while, for, and for/in. One of the following subsections is devoted to each of them. One common use of loops is to traverse the elements of an array.
while loop
The if statement is the basic conditional statement in JavaScript, and the basic loop for JavaScript is the while loop. It has the following syntax:
while (expression) (instruction)
The while loop begins by evaluating an expression. If this expression evaluates to false, the interpreter skips the statement that makes up the body of the loop and moves on to the next statement in the program. If the expression evaluates to true, then the statement that forms the body of the loop is executed, then control is transferred to the beginning of the loop and the expression is evaluated again. In other words, the interpreter executes the loop body instruction over and over again as long as the value of the expression remains true. Please note that it is possible to create an infinite loop using the while(true) syntax.
Typically you don't want the JavaScript interpreter to perform the same operation over and over again. In almost every loop, with each iteration of the loop, one or more variables change their values. Because the variable changes, what the instruction does may differ each time it passes through the loop body.
Additionally, if the variable(s) being modified are present in the expression, the value of the expression may change with each pass of the loop. This is important because otherwise the expression whose value was true will never change and the loop will never end! Below is an example of a while loop that prints the numbers from 0 to 9:
Var count = 0; while (count
As you can see, the count variable is set to 0 at the beginning, and then its value is incremented each time the body of the loop is executed. After the loop has been executed 10 times, the expression will return false (that is, the count variable is no longer less than 10), the while statement will end, and the interpreter will move on to the next statement in the program. Most loops have counter variables similar to count. Most often, the variables named i, j, and k act as loop counters, although in order to make the program code more understandable, you should give the counters more descriptive names.
do/while loop
A do/while loop is similar in many ways to a while loop, except that the loop expression is tested at the end rather than at the beginning. This means that the body of the loop is always executed at least once. This instruction has the following syntax:
do (statement) while (expression);
The do/while loop is used less frequently than its sister while loop. The fact is that in practice, the situation when you are sure in advance that you will need to execute the body of the loop at least once is somewhat unusual. Below is an example of using a do/while loop:
Function printArray(a) ( var len = a.length, i = 0; if (len == 0) console.log("Empty array"); else ( do ( console.log(a[i]); ) while (++i
There are two differences between a do/while loop and a regular while loop. First, a do loop requires both the do keyword (to mark the start of the loop) and the while keyword (to mark the end of the loop and specify a condition). Second, unlike a while loop, a do loop ends with a semicolon. A while loop does not need to end with a semicolon if the body of the loop is enclosed in curly braces.
for loop
A for loop is a loop construct that is often more convenient than a while loop. The for loop makes it easy to construct loops that follow a pattern common to most loops. Most loops have some kind of counter variable. This variable is initialized before the loop starts and is checked before each iteration. Finally, the counter variable is incremented or otherwise modified at the end of the loop body, just before the variable is checked again. Initialization, verification, and update are the three key operations performed on a loop variable. The for statement makes these three steps explicit part of the loop syntax:
for(initialization; checking; increment) (instruction)
Initialize, check, and increment are three expressions (separated by semicolons) that are responsible for initializing, checking, and incrementing a loop variable. Placing them on the first line of the loop makes it easier to understand what the for loop is doing and prevents you from forgetting to initialize or increment a loop variable.
The easiest way to explain the for loop is to show the equivalent while loop:
initialization; while(check) ( instruction; increment; )
In other words, the initialization expression is evaluated once before the loop begins. This expression is typically an expression with side effects (usually an assignment). JavaScript also allows the initialization expression to be a var variable declaration statement, so it is possible to declare and initialize a loop counter at the same time.
The test expression is evaluated before each iteration and determines whether the body of the loop will be executed. If the result of the test is true, the instruction that is the body of the loop is executed. At the end of the loop, the increment expression is evaluated. For this expression to be meaningful, it must be an expression with side effects. Typically this is either an assignment expression or an expression using the ++ or -- operator.
You can also print the numbers 0 through 9 using a for loop, as shown below, as opposed to the equivalent while loop shown in the example earlier:
For (var count = 0; count
Of course, loops can be much more complex than these simple examples, and sometimes multiple variables change in each iteration of the loop. This situation is the only time in JavaScript where the comma operator is often used - it allows you to combine multiple initialization and incrementing expressions into a single expression suitable for use in a for loop:
Var i,j; for (i = 0, j = 0; i
for/in loop
The for/in loop uses the for keyword, but it is completely different from a regular for loop. The for/in loop has the following syntax:
for (variable in object) (statement)
The variable here is usually the name of the variable, but you can also use the var statement, which declares a single variable. The object parameter is an expression that returns an object. And as usual, an instruction is an instruction or block of instructions that forms the body of a loop.
To traverse the elements of an array, it is natural to use a regular for loop:
Var arr = ; for (var i = 0; i
The for/in statement also naturally allows you to traverse the properties of an object:
// Create a new object var obj = (name:"Alex", password:"12345" ); for (var i in obj) ( // Print the value of each object property console.log(obj[i]); )
To execute a for/in statement, the JavaScript interpreter first evaluates the expression object. If it returns null or undefined, the interpreter skips the loop and moves on to the next statement. If the expression returns a simple value, it is converted to an equivalent wrapper object. Otherwise, the expression returns an object. The interpreter then executes one iteration of the loop for each enumerable property of the object. Before each iteration, the interpreter evaluates the value of the expression, stores it in a variable, and assigns it a property name (a string value).
