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JavaScript Loops

JavaScript performs several types of repetitive operations, called "looping". Loops are set of instructions used to repeat the same block of code till a specified condition returns false or true depending on how you need it. To control the loops you can use counter variable that increments or decrements with each repetition of the loop.

JavaScript supports different kinds of loops: for - The for statements are best used when you want to perform a loop a

specific number of times.

for/in - loops through the properties of an object while - The while statements are best used to perform a loop an undetermined

number of times.

do/while - loops through a block of code while a specified condition is true

Why Loops and what are the uses of Loops?

Very often when you write code, you want the same block of code to run a number of times. You can use looping statements in your code to do this. Loops are handy, if you want to run the same code over and over again, each time with a different value. You can use loops.

Tips to improve performance of loops:

a. How its Typically Written? I think its safe to say that most beginners to intermediate JavaScript developers will write forloop like this: var anchors = document.getElementsByTagName("a"); for (i=0;i

b. Fix the spacing : Heres how our code looks after correcting all the spacing issues: var anchors = document.getElementsByTagName("a"); for (i = 0; i < anchors.length; i++) { // do some stuff here } Technically, I didnt need to put a space after the semicolons, but I did it anyhow to aid readability. In addition to proper spacing around the operators, JSLint also requires a space between the closing parenthesis and the opening curly brace. All these spacing issues are from what I understand, to help avoid using confusing code thats prone to accidental errors or code in which errors are hard to spot.

c. Localize your variable: After fixing the spacing issues, we can now focus on another error presented by JSLint: the global variable i. Any variable not defined using the var statement in JavaScript is global in scope. This is bad practice, and it can be easily overlooked inside of such a commonly-used bit of code. So lets make our variable local using the var keyword. We could do this a couple of ways, but the following method will suffice to make JSLint happy: var anchors = document.getElementsByTagName("a"); for (var i = 0; i < anchors.length; i++) { // do some stuff here }

d. Dont calculate length on each iteration : As the code is now, the length of anchors is calculated on each loop iteration. In a large application, and with large values and multiple loops, this can contribute to performance issues. So although in many small instances this might not matter, it is best practice to try to cache values before using them. So we can alter the code to look like this instead: var anchors = document.getElementsByTagName("a"); for (var i = 0, j = anchors.length; i < j; i ++) { // do some stuff here } Now the value gets calculated only once, and is stored in the variable j.

The For Loop

The JavaScript for loop repeats a series of statements any number of times and includes an optional loop counter that can be used in the execution of the statements. The following is the formal syntax definition: for ( [initial expression]; [condition]; [update expression]) { //statements } When the, for loop executes, the following occurs: 1. The initializing expression is executed. This expression usually initializes

one or more loop counters, but the syntax allows an expression of any degree of complexity.

2. The condition expression is evaluated. If the value of condition is true, the loop statements execute. If the value of condition is false, the for loop terminates.

3. The update expression increment executes. 4. The statements execute, and control returns to step 2.

Example Click function myFunction() { var text = ""; for (var i = 0; i < 5; i++) { text += "The number is " + i + ""; } document.getElementById("demo").innerHTML = text; }

Output:

The number is 0 The number is 1 The number is 2 The number is 3 The number is 4 From the example above, you can read: Statement 1 sets a variable before the loop starts (var i = 0). Statement 2 defines the condition for the loop to run (i must be less than 5). Statement 3 increases a value (i++) each time the code block in the loop has been executed.

if/else statement within a for loop

If/else statements allow an action to be carried out if a particular condition is matched, else a different action will be carried out. So if an if/else statement is placed within a loop, itll run each time the loop does. The example below shows how the numbers printed from a counter for loop can be manipulated. I want the loop to tell me which numbers are odd and which are even; if the number is even, I want (even) to be printed after the number and if it is odd, I want (odd) to be printed. Here is the code that can accomplish this and the result the loop is counting from 1 to 10 and checking each time it is run to see if the variable value passed in is divisible by 2 (if it is, it is an even number).

Example

// Create the for loop for (var i = 1; i

Nested for loops For loops can also be nested inside of each other. In my simple example below, imagine that I want to print out a list that I can record my workouts on in the gym. I want to do four exercises with three sets each, so Ill need a list that will reflect this. I can use a for loop nested within a for loop to achieve this. Ill need the first loop to run four times, and each time it runs, to print out an exercise number. The loop within it will run three times to create the required number of sets within each exercise I want to complete so I can tick them off as I do them.

Example // Create the first for loop for (var i = 1; i

Output:

Exercise 1: Set 1 Set 2 Set 3 Exercise 2: Set 1 Set 2 Set 3 Exercise 3: Set 1 Set 2 Set 3 Exercise 4: Set 1 Set 2 Set 3

for loop within a function A for loop can be placed inside a function. This way, parameters can be passed into the function to be used in the loop. I am going to illustrate this by creating a loop within a function which will print times tables of any number from 1x to 5x. In this case it will print the 12 times table.

Example // Create the function var timesTable = function(number) { // Create the for loop for (var i = 1; i

Output:

12 times 1 equals 12 12 times 2 equals 24 12 times 3 equals 36 12 times 4 equals 48 12 times 5 equals 60

for loop using an array

An array stores multiple pieces of data at the same time. A for loop can be used to item in an array one by one. My example below shows how I can print out a list of every flavor cake that I like. I use an array to store the cake flavors for later use. Each time the loop runs, it looks at each array item in turn and prints out I like *arrayitem* cake.

Example

// Create the array var flavours = ["chocolate", "ginger", "carrot", "coffee", "walnut", "banana"]; // Create the for loop for (var i = 0, flen = flavours.length; i < flen; i ++) { console.log("I like " + flavours[i] + " cake"); }

Output:

I like chocolate cake I like ginger cake I like carrot cake I like coffee cake I like walnut cake I like banana cake

for loop using an array within a function This example is very similar to the one above, so the way the loop is interacting with the array is exactly the same. However, it is placed within a function and can be called as such, with arguments passed in.

Example // Create the array var flavours = ["chocolate", "ginger", "carrot", "coffee", "walnut", "banana"]; // Create the function var cake = function(singleFlavour) { // Create the for loop for (var i = 0, flen = flavours.length; i < flen; i ++) { console.log("I like " + singleFlavour[i] + " cake"); } }; cake(flavours);

Output:

I like chocolate cake I like ginger cake I like carrot cake I like coffee cake I like walnut cake I like banana cake

The For/In Loop

JavaScript includes a variation of the for loop, called a for-in loop, which has special powers of extracting the names and values of any object property currently in the browsers memory. The syntax looks like this:

for (var in object) { //statements } The object parameter is not the string name of an object but a reference to the object itself. JavaScript delivers an object reference if you provide the name of the object as an unquoted string, such as window or document. Using the var variable, you can create a script that extracts and displays the range of properties for any given object.

Example

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

Output:

John Doe 25

The While Loop

The for loop is not the only kind of repeat loop you can construct in JavaScript. Another statement, called a while statement, sets up a loop in a slightly different format. Rather than providing a mechanism for modifying a loop counter, a while repeat loop assumes that your script statements will reach a condition that forcibly exits the repeat loop. The basic syntax for a while loop is

while (condition) { //statements } The condition expression is the same kind that you saw in the middle parameter of the for loop. You introduce this kind of loop if some condition exists in your code (evaluates to true) before reaching this loop. The loop then performs some action, which affects that condition repeatedly until that condition becomes false. At that point, the loop exits, and script execution continues with statements after the closing brace. If the statements inside the while loop do not somehow affect the values being tested in condition, your script never exits, and it becomes stuck in an infinite loop.

Example Click function myFunction() { var text = ""; var i = 0; while (i < 5) { text += "The number is " + I + ; i++; } document.getElementById("demo").innerHTML = text; }

Output:

The number is 0 The number is 1 The number is 2 The number is 3 The number is 4 Note: If you forget to increase the variable used in the condition, the loop will never end. This will crash your browser.

The Do/While Loop

The do...while loop is similar to the while loop except that the condition check happens at the end of the loop. This means that the loop will always be executed at least once, even if the condition is false. Note the semicolon used at the end of the do...while loop. An important difference distinguishes the do-while loop from the while loop. In the do-while loop, the statements in the construction always execute at least one time before the condition can be tested; in a while loop, the statements may never execute if the condition tested at the outset evaluates to false. So, just think of the do-while loop as a while loop where the first statement gets executed no matter what. Use a do-while loop when you know for certain that the looped statements are free to run at least one time. If the condition may not be met the first time, use the while loop. For many instances, the two constructions are interchangeable.

do{ code block to be executed }while(condition);

Example Click function myFunction() { var text = "" var i = 0; do { text += "The number is " + i; i++; } while (i < 5); document.getElementById("demo").innerHTML = text; }

Output:

The number is 0 The number is 1 The number is 2 The number is 3 The number is 4

The Break Statement Some loop constructions perform their job as soon as a certain condition is met, at which point they have no further need to continue looping through the rest of the values in the loop counters range. A common scenario for this is the cycling of a loop through an entire array in search of a single entry that matches some criterion. That criterion test is set up as an if construction inside the loop. If that criterion is met, you break out of the loop and let the script continue with the more meaningful processing of succeeding statements in the main flow. To accomplish that exit from the loop, use the break statement. The break statement breaks the loop and continues executing the code after the loop (if any):

Example var text = ""; var i; for (i = 0; i < 10; i++) { if (i === 3) { break; } text += "The number is " + i + ""; } document.getElementById("demo").innerHTML = text;

Output: The number is 0 The number is 1 The number is 2

The Continue Statement

One other possibility in a for loop is that you may want to skip execution of the nested statements for just one condition. In other words, as the loop goes merrily on its way round and round, executing statements for each value of the loop counter, one value of that loop counter may exist for which you dont want those statements to execute. To accomplish this task, the nested statements need to include an if construction to test for the presence of the value to skip. When that value is reached, the continue command tells JavaScript to immediately skip the rest of the body, execute the update statement, and loop back around to the top of the loop.

Example

var text = ""; var i; for (i = 0; i < 10; i++) { if (i === 3) { continue; } text += "The number is " + i + ""; } document.getElementById("demo").innerHTML = text;

Output:

The number is 0 The number is 1 The number is 2 The number is 4 The number is 5 The number is 6 The number is 7 The number is 8 The number is 9 You can see the above example skips the value of 3.

Summary

HI, myself Akhilesh Ojha a post graduate in computer science and Web UI developer. I am excelling my JavaScript, jQuery and AngularJS skills in ADMEC Multimedia Institute currently on weekends. I was given this to write as a classroom project and I tried my best to explain Loops in JavaScript with examples. All examples are tested and working fine. In this article, I explained Loops introduction, types of loops, why loops, uses of loops, tips to improve performance of loops, for loop, for...in loop, while loop, do...while loop, break and continue statement.

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