A browser-ready efficient scrolling container based on UITableView

A browser-ready efficient scrolling container based on UITableView

A browser-ready efficient scrolling container based on UITableView .React Infinite 0.7.1 only supports React 0.14 and above. Please pin your package to 0.6.0 for React 0.13 support.

React Infinite 0.7.1 only supports React 0.14 and above. Please pin your package to 0.6.0 for React 0.13 support.

When a long list of DOM elements are placed in a scrollable container, all of them are kept in the DOM even when they are out the user's view. This is highly inefficient, especially in cases when scrolling lists can be tens or hundreds of thousands of items long. React Infinite solves this by rendering only DOM nodes that the user is able to see or might soon see. Other DOM nodes are clustered and rendered as a single blank node.

Installation

In the Browser

The relevant files are dist/react-infinite.js and dist/react-infinite.min.js. You must have React available as a global variable named React on the window. Including either file, through concatenation or a script tag, will produce a global variable named Infinite representing the component.

In NPM

React Infinite uses a Universal Module Definition so you can use it in NPM as well. npm install this package and

source-js
var Infinite = require('react-infinite');

In Browserify

If you want to use the source with Browserify, the ES5-compiled source is directly requirable from the /build folder off NPM.

Otherwise, you can follow the instructions for NPM.

Basic Use

Elements of Equal Height

To use React Infinite with a list of elements you want to make scrollable, provide them to React Infinite as children.

text-xml
<Infinite containerHeight={200} elementHeight={40}>
    <div className="one"/>
    <div className="two"/>
    <div className="three"/>
</Infinite>
Elements of Varying Heights

If not all of the children have the same height, you must provide an array of integers to the elementHeight prop instead.

text-xml
<Infinite containerHeight={200} elementHeight={[111, 252, 143]}>
    <div className="111-px"/>
    <div className="252-px"/>
    <div className="143-px"/>
</Infinite>
Using the Window to Scroll (useWindowAsScrollContainer mode)

To use the entire window as a scroll container instead of just a single div (thus using window.scrollY instead of a DOM element's scrollTop), add the useWindowAsScrollContainer prop.

text-xml
<Infinite containerHeight={200} elementHeight={[111, 252, 143]}
          useWindowAsScrollContainer>
    <div className="111-px"/>
    <div className="252-px"/>
    <div className="143-px"/>
</Infinite>

As A Chat or Message Box (displayBottomUpwards mode)

React Infinite now supports being used as a chat box, i.e. appended elements appear at the bottom when added, and the loading of the next page occurs when the user scrolls to the top of the container. To do so, simply add the displayBottomUpwards prop. A sample implementation can be consulted for more information - run gulp develop to compile the example files.

text-xml
<Infinite containerHeight={200} elementHeight={[111, 252, 143]}
          displayBottomUpwards>
    // insert messages for subsequent pages at this point
    <div className="third-latest-chat"/>
    <div className="second-latest-chat"/>
    <div className="latest-chat-message"/>
</Infinite>

Note on Smooth Scrolling

A wrapper div is applied that disables pointer events on the children for a default of 150 milliseconds after the last user scroll action for browsers with inertial scrolling. To configure this, set timeScrollStateLastsForAfterUserScrolls.

Static Methods Function Infinite.containerHeightScaleFactor(Number number)

This function allows a value to be specified for preloadBatchSize and preloadAdditionalHeight that is a relative to the container height. Please see the documentation for those two configuration options for further information on how to use it.

Configuration Options

Children

The children of the <Infinite> element are the components you want to render. This gives you as much flexibility as you need in the presentation of those components. Each child can be a different component if you desire. If you wish to render a set of children not all of which have the same height, you must map each component in the children array to an number representing its height and pass it in as the elementHeight prop.

Major Display Modes

By default, React Infinite renders a single element with the provided containerHeight, and the list sticks to the top like a regular table. However, you can choose to use the entire window as the scroll container or make React Infinite like a chatbox with the following options. They can be used together if you wish.

Bool useWindowAsScrollContainer

Defaults to false. This option allows the window to be used as the scroll container, instead of an arbitrary div, when it is set to true. This means that scroll position is detected by window.scrollY instead of the scrollTop of the div that React Infinite creates. Using this option is a way of achieving smoother scrolling on mobile before the problem is solved for container divs.

Bool displayBottomUpwards

Defaults to false. This allows React Infinite to be used as a chatbox. This means that the scroll is stuck to the bottom by default, and the user scrolls up to the top of the container to load the next page. The children are displayed in the same order.

Configuration Options

(Required) Number | [Number] elementHeight

If each child element has the same height, you can pass a number representing that height as the elementHeight prop. If the children do not all have the same height, you can pass an array which is a map the children to numbers representing their heights to the elementHeight prop.

Number containerHeight

The height of the scrolling container in pixels. This is a required prop if useWindowAsScrollContainer is not set to true.

Number | Object preloadBatchSize

Defaults to this.props.containerHeight * 0.5. Imagine the total height of the scrollable divs. Now divide this equally into blocks preloadBatchSize pixels high. Every time the container's scrollTop enters each of these blocks the set of elements rendered in full are those contained within the block and elements that are within preloadAdditionalHeight above and below it.

When working with the window as the scroll container, it is sometimes useful to specify a scale factor relative to the container height as the batch size, so your code does not need to know anything about the window. To do this, use Infinite.containerHeightScaleFactor. So, for example, if you want the preloaded batch size to be twice the container height, write preloadBatchSize={Infinite.containerHeightScaleFactor(2)}.

Number | Object preloadAdditionalHeight

Defaults to this.props.containerHeight. The total height of the area in which elements are rendered in full is height of the current scroll block (see preloadBatchSize) as well as preloadAdditionalHeight above and below it.

When working with the window as the scroll container, it is sometimes useful to specify this relative to the container height. If you want the preloaded additional height to be twice the container height, write preloadAdditionalHeight={Infinite.containerHeightScaleFactor(2)}. Please see preloadBatchSize for more details.

Function handleScroll(DOMNode node)

Defaults to function(){}. A function that is called when the container is scrolled, i.e. when the onScroll event of the infinite scrolling container is fired. The only argument passed to it is the native DOM Node of the scrolling container.

Number infiniteLoadBeginEdgeOffset

Defaults to undefined, which means that infinite loading is disabled. To disable infinite loading, do not provide this property or set it to undefined.

Regular Mode When the user reaches this number of pixels from the bottom, the infinite load sequence will be triggered by showing the infinite load spinner delegate and calling the function onInfiniteLoad.

displayBottomUpwards mode When the user reaches this number of pixels from the top of the container, the infinite load sequence will be triggered by showing the infinite loading spinner delegate at the top of the container and calling onInfiniteLoad.

Function onInfiniteLoad()

Defaults to function(){}. This function is called when the scroll exceeds infiniteLoadBeginEdgeOffset. Before this function is called, the infinite loading spinner is automatically turned on. You can set up infinite scrolling with this function like this:

  1. Fetch a new page of records from the appropriate API
  2. When the AJAX call returns, send the new list of elements (with the items that were just fetched) back as the children of React Infinite.
  3. Set React Infinite's isInfiniteLoading prop to false to hide the loading spinner display

onInfiniteLoad relies heavily on passing props as a means of communication in the style of idiomatic React.

React Node loadingSpinnerDelegate

Defaults to <div/>. The element that is provided is used to render the loading view when React Infinite's isInfiniteLoading property is set to true. A React Node is anything that satisfies React.PropTypes.node.

Bool isInfiniteLoading

Defaults to false. This property determines whether the infinite spinner is showing.

Number timeScrollStateLastsForAfterUserScrolls

Defaults to 150 (in milliseconds). On Apple and some other devices, scroll is inertial. This means that the window continues to scroll for several hundred milliseconds after an onScroll event is fired. To prevent janky behavior, we do not want pointer-events to reactivate before the window has finished moving. Setting this parameter causes the Infinite component to think that the user is still scrolling for the specified number of milliseconds after the last onScroll event is received.

String className

Allows a CSS class to be set on the scrollable container.

Sample Code

Code samples are now available in the /examples directory for your perusal. Two examples are provided, one for constant height with infinite loading and another with random variable heights with infinite loading. To generate the files necessary for the examples, execute npm install && gulp build -E. You may need to first install gulp with npm install -g gulp.

To get you started, here is some sample code that implements an infinite scroll with an simulated delay of 2.5 seconds. A live demo of this example is available on our blog.

source-js
var createReactClass = require('create-react-class');

var ListItem = createReactClass({
    render: function() {
        return <div className="infinite-list-item">
        List Item {this.props.num}
        </div>;
    }
});

var InfiniteList = createReactClass({
    getInitialState: function() {
        return {
            elements: this.buildElements(0, 20),
            isInfiniteLoading: false
        }
    },

    buildElements: function(start, end) {
        var elements = [];
        for (var i = start; i < end; i++) {
            elements.push(<ListItem key={i} num={i}/>)
        }
        return elements;
    },

    handleInfiniteLoad: function() {
        var that = this;
        this.setState({
            isInfiniteLoading: true
        });
        setTimeout(function() {
            var elemLength = that.state.elements.length,
                newElements = that.buildElements(elemLength, elemLength + 1000);
            that.setState({
                isInfiniteLoading: false,
                elements: that.state.elements.concat(newElements)
            });
        }, 2500);
    },

    elementInfiniteLoad: function() {
        return <div className="infinite-list-item">
            Loading...
        </div>;
    },

    render: function() {
        return <Infinite elementHeight={40}
                         containerHeight={250}
                         infiniteLoadBeginEdgeOffset={200}
                         onInfiniteLoad={this.handleInfiniteLoad}
                         loadingSpinnerDelegate={this.elementInfiniteLoad()}
                         isInfiniteLoading={this.state.isInfiniteLoading}
                         >
            {this.state.elements}
        </Infinite>;
    }
});

ReactDOM.render(<InfiniteList/>, document.getElementById('react-example-one'));

SeatGeek also currently uses React Infinite in production on our event pages; because we only have pages for events in the future, a link would not be appropriate. To see one, head to one of our team pages for the New York Giants, or the New York Mets, or the New York Knicks, and click on the green button for an event to see them in action in the Omnibox.

Download Details:

Author: seatgeek

Official Website: https://github.com/seatgeek/react-infinite

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ReactJS vs Angular vs Vue: Best Javascript Framework For Your Project

ReactJS vs Angular vs Vue: Best Javascript Framework For Your Project

ReactJS vs Angular vs Vue: Best Javascript Framework For Your Project. This video covers the key differences between ReactJS, Angular and Vue with respect to the following: Use case, Performance, Data binding, Scripting language, Testing, Community support, Growth curve

ReactJS, Angular and Vue have been all the craze the past couple of years. All these tools help developers build fancy user interfaces for web and mobile applications. But if all of them serve the same objective, what do you think makes them stand apart! Well, to address that, this video covers the key differences between ReactJS, Angular and Vue with respect to the following:

  1. Use case 03:11
  2. Performance 03:30
  3. Data binding 04:52
  4. Scripting language 06:05
  5. Testing 06:52
  6. Community support 07:31
  7. Growth curve 08:24

What is JavaScript – All You Need To Know About JavaScript

What is JavaScript – All You Need To Know About JavaScript

In this article on what is JavaScript, we will learn the basic concepts of JavaScript.

After decades of improvement, JavaScript has become one of the most popular programming languages of all time. It all started in the year 1995 when Brendan Eich created JavaScript in a span of 10 days. Since then, it has seen multiple versions, updates and has grown to the next level.

Here’s a list of topics that I’ll be covering in this blog:

  1. What is JavaScript
  2. What can JavaScript do?
  3. JavaScript Frameworks
  4. The Big Picture: HTML, CSS & JavaScript
  5. Benefits of JavaScript
  6. Fundamentals of JavaScript
    VariablesConstantsData TypesObjectsArraysFunctionsConditional statementsLoopsSwitch case
What is JavaScript?

JavaScript is a high level, interpreted, programming language used to make web pages more interactive.

Have you ever thought that your website is missing something? Maybe it’s not engaging enough or it’s not as creative as you want it to be. JavaScript is that missing piece which can be used to enhance web pages, applications, etc to provide a more user-friendly experience.

What is JavaScript?

JavaScript is the language of the web, it is used to make the web look alive by adding motion to it. To be more precise, it’s a programming language that let’s you implement complex and beautiful things/design on web pages. When you notice a web page doing more than just sit there and gawk at you, you can bet that the web page is using JavaScript.

Feature of JavaScript

Scripting language and not Java: In fact, JavaScript has nothing to do with Java. Then why is it called “Java” Script? When JavaScript was first released it was called Mocha, it was later renamed to LiveScript and then to JavaScript when Netscape (founded JavaScript) and Sun did a license agreement. Object-based scripting language which supports polymorphism, encapsulation and to some extent inheritance as well.**Interpreted language: **It doesn’t have to be compiled like Java and C which require a compiler.JavaScript runs in a browser: You can run it on Google Chrome, Internet Explorer, Safari, etc. JavaScript can execute not only in the browser but also on the server and any device which has a JavaScript Engine.

What is JavaScript – Stackoverflow stats

Currently, we have 100s of programming languages and every day new languages are being created. Among these are few powerful languages that bring about big changes in the market and JavaScript is one of them.

JavaScript has always been on the list of popular programming languages. According to StackOverflow, for the 6th year in a row, JavaScript has remained the most popular and commonly used programming language.

What can JavaScript do?

JavaScript is mainly known for creating beautiful web pages & applications. An example of this is Google Maps. If you want to explore a specific map, all you have to do is click and drag with the mouse. And what sort of language could do that? You guessed it! It’s JavaScript.JavaScript can also be used in smart watches. An example of this is the popular smartwatch maker called Pebble. Pebble has created Pebble.js which is a small JavaScript Framework that allows a developer to create an application for the Pebble line of watches in JavaScript.

What is JavaScript – Applications of JavaScript
Most popular websites like Google, Facebook, Netflix, Amazon, etc make use of JavaScript to build their websites.Among things like mobile applications, digital art, web servers and server applications, JavaScript is also used to make Games. A lot of developers are building small-scale games and apps using JavaScript.## JavaScript Frameworks

One major reason for the popularity of JavaScript is the JavaScript Frameworks. Here’s a brief introduction of the most trending JavaScript frameworks :

  1. AngularJS is Google’s web development framework which provides a set of modern development and design features for rapid application development.

  2. ReactJS is another top JavaScript framework mainly maintained by Facebook and it’s behind the User Interface of Facebook and Instagram, showing off its efficiency in maintaining such high traffic applications.

What is JavaScript – JavaScript Frameworks

  1. MeteorJS is mainly used for providing back-end development. Using JavaScript on the back-end to save time and build expertise is one of the major ideas behind Meteor.

  2. jQuery can be used when you want to extend your website and make it more interactive. Companies like Google, WordPress and IBM rely on jQuery.

The Big Picture: HTML, CSS & JavaScript

Anyone familiar with JavaScript knows that it has something to do with HTML and CSS. But what is the relationship between these three? Let me explain this with an analogy.

What is JavaScript – HTML, CSS and JavaScript

Think of HTML (HyperText Markup Language) as the skeleton of the web. It is used for displaying the web.

On the other hand, CSS is like our clothes. We put on fashionable clothes to look better. Similarly, the web is quite stylish as well. It uses CSS which stands for Cascading Style Sheets for styling purpose.

Then there is JavaScript which puts life into a web page. Just like how kids move around using the skateboard, the web also motions with the help of JavaScript.

Benefits of JavaScript

There has to be a reason why so many developers love working on JavaScript. Well, there are several benefits of using JavaScript for developing web applications, here’s a few benefits:

It’s easy to learn and simple to implement. It is a weak-type programming language unlike the strong-type programming languages like Java and C++, which have strict rules for coding.

It’s all about being fast in today’s world and since JavaScript is mainly a client-side programming language, it is very fast because any code can run immediately instead of having to contact the server and wait for an answer.

Rich set of frameworks like AngularJS, ReactJS are used to build web applications and perform different tasks.

**Builds interactive websites: **We all get attracted to beautifully designed websites and JavaScript is the reason behind such attractive websites and applications.

JavaScript is an interpreted language that does not require a compiler because the web interprets JavaScript. All you need is a browser like Google Chrome or Internet Explorer and you can do all sorts of stuff in the browser.

JavaScript is platform independent and it is supported by all major browsers like Internet Explorer, Google Chrome, Mozilla Firefox, Safari, etc.

JavaScript Fundamentals

In this What is JavaScript blog, we’ll cover the following basic fundamentals of JavaScript
VariablesConstantsData TypesObjectsArraysFunctionsConditional statementsLoopsSwitch case## Variables

Variable is a name given to a memory location which acts as a container for storing data temporarily. They are nothing but reserved memory locations to store values.

What is JavaScript – Variables

To declare a variable in JavaScript use the ‘let’ keyword. For example:

let age;
age=22;

In the above example, I’ve declared a variable ‘age’ by using the ‘let’ keyword and then I’ve stored a value (22) in it. So here a memory location is assigned to the ‘age’ variable and it contains a value i.e. ’22’.

Constants

Constants are fixed values that don’t change during execution time.

To declare a constant in JavaScript use the ‘const’ keyword. For example:

const mybirthday;
mybirthday='3rd August'; 

Data types

You can assign different types of values to a variable such as a number or a string. In JavaScript, there are two categories of data types :

What is JavaScript – Data Types

Objects

An object is a standalone entity with properties and types and it is a lot like an object in real life. For example, consider a girl, whose name is Emily, age is 22 and eye-color is brown. In this example the object is the girl and her name, age and eye-color are her properties.

What is JavaScript – Objects example

Objects are variables too, but they contain many values, so instead of declaring different variables for each property, you can declare an object which stores all these properties.

To declare an object in JavaScript use the ‘let’ keyword and make sure to use curly brackets in such a way that all property-value pairs are defined within the curly brackets. For example:

let girl= {
name: 'Emily',
age: 22,
eyeColour: 'Brown'
};

In the above example, I’ve declared an object called ‘girl’ and it has 3 properties (name, age, eye colour) with values (Emily, 22, Brown).

Arrays

An array is a data structure that contains a list of elements which store multiple values in a single variable.

For example, let’s consider a scenario where you went shopping to buy art supplies. The list of items you bought can be put into an array.

What is JavaScript – Arrays example

To declare an array in JavaScript use the ‘let’ keyword with square brackets and all the array elements must be enclosed within them. For example:

let shopping=[];
shopping=['paintBrush','sprayPaint','waterColours','canvas'];

In the above example I’ve declared an array called ‘shopping’ and I’ve added four elements in it.

Also, array elements are numbered from zero. For example this is how you access the first array element:

shopping[0];		

Functions

A function is a block of organised, reusable code that is used to perform single, related action.

Let’s create a function that calculates the product of two numbers.

To declare a function in JavaScript use the ‘function’ keyword. For example:

function product(a, b) {
return a*b;
}

In the above example, I’ve declared a function called ‘product’ and I’ve passed 2 parameters to this function, ‘a’ and ‘b’ which are variables whose product is returned by this function. Now, in order to call a function and pass a value to these parameters you’ll have to follow the below syntax:

product(8,2);

In the above code snippet I’m calling the product function with a set of values (8 & 2). These are values of the variables ‘a’ and ‘b’ and they’re called as arguments to the function.

Conditional statements – if

Conditional statement is a set of rules performed if a certain condition is met. The ‘if’ statement is used to execute a block of code, only if the condition specified holds true.

What is JavaScript – if flowchart

To declare an if statement in JavaScript use the ‘if’ keyword. The syntax is:

if(condition) {
statement;
}

Now let’s look at an example:

let numbers=[1,2,1,2,3,2,3,1];
if(numbers[0]==numbers[2]) {
console.log('Correct!');
}

In the above example I’ve defined an array of numbers and then I’ve defined an if block. Within this block is a condition and a statement. The condition is ‘(numbers[0]==numbers[2])’ and the statement is ‘console.log(‘Correct!’)’. If the condition is met, only then the statement will be executed.

Conditional statements- Else if

Else statement is used to execute a block of code if the same condition is false.

What is JavaScript – Else-if flowchart

The syntax is:

if(condition) {
statement a;
}
else (condition) {
statement b;
}

Now let’s look at an example:

let numbers=[1,2,1,2,3,2,3,1];
if(numbers[0]==numbers[4] {
console.log("Correct!");
}
else {
console.log("Wrong, please try again");
}

In the above example, I’ve defined an if block as well as an else block. So if the conditions within the if block holds false then the else block gets executed. Try this for yourself and see what you get!

**Loops **

Loops are used to repeat a specific block until some end condition is met. There are three categories of loops in JavaScript :

  1. while loop
  2. do while loop
  3. for loop
While loop

While the condition is true, the code within the loop is executed.

What is JavaScript – while loop flowchart

The syntax is:

while(condition) {
loop code;
}

Now let’s look at an example:

let i=0;
while(i < 5) {
console.log("The number is " +i);
i++;
}

In the above example, I’ve defined a while loop wherein I’ve set a condition. As long as the condition holds true, the while loop is executed. Try this for yourself and see what you get!

Do while loop

This loop will first execute the code, then check the condition and while the condition holds true, execute repeatedly.

What is JavaScript – Do while loop flowchart

Refer the syntax to better understand it:

do {
loop code;
} while(condition);

This loop executes the code block once before checking if the condition is true, then it will repeat the loop as long as the condition holds true.

Now let’s look at an example:

do {
console.log("The number is " +i);
i++;
}
while(i > 5);

The above code is similar to the while loop code except, the code block within the do loop is first executed and only then the condition within the while loop is checked. If the condition holds true then the do loop is executed again.

For loop

The for loop repeatedly executes the loop code while a given condition is TRUE. It tests the condition before executing the loop body.

What is JavaScript – for loop flowchart

The syntax is:

for(begin; condition; step) {
loop code;
}

In the above syntax:

  • begin statement is executed one time before the execution of the loop code
  • condition defines the condition for executing the loop code
  • step statement is executed every time after the code block has been executed

For example:

for (i=0;i<5;i++) {
console.log("The number is " +i);
}

In the above example, I’ve defined a for loop within which I’ve defined the begin, condition and step statements. The begin statement is that ‘i=0’. After executing the begin statement the code within the for loop is executed one time. Next, the condition is checked, if ‘i<5’ then, the code within the loop is executed. After this, the last step statement (i++) is executed. Try this and see what you get!

Switch Case

The switch statement is used to perform different actions based on different conditions.

What is JavaScript – Switch case flowchart

Let’s look at the syntax for switch case:

switch(expression) {
case 1:
code block 1
break;
case 2:
code block 2
break;
default:
code block 3
break;
}

How does it work?

  • Switch expression gets evaluated once
  • Value of the expression is compared with the values of each case
  • If there is a match, the associated block of code is executed

Let’s try this with an example:

let games='football';
switch(games) {
case "throwball":
console.log("I dislike throwball!");
break;
case "football":
console.log("I love football!");
break;
case "cricket":
console.log("I'm a huge cricket fan!");
break;
default:
console.log("I like other games");
break;
}

In the above example the switch expression is ‘games’ and the value of games is ‘football’. The value of ‘games’ is compared with the value of each case. In this example it is compared to ‘throwball’, ‘cricket’ and ‘football’. The value of ‘games’ matches with the case ‘football’, therefore the code within the ‘football’ case is executed. Try this for yourself and see what you get!

With this, we come to the end of this blog. I hope you found this blog informative and I hope you have a basic understanding of JavaScript. In my next blog on JavaScript I’ll be covering in-depth concepts, so stay tuned.

Also, check out our video on JavaScript Fundamentals if you want to get started as soon as possible and don’t forget to leave a comment if you have any doubt and also, let us know whether you’d want us to create more content on JavaScript. We are listening!

JavaScript Tutorial: if-else Statement in JavaScript

JavaScript Tutorial: if-else Statement in JavaScript

This JavaScript tutorial is a step by step guide on JavaScript If Else Statements. Learn how to use If Else in javascript and also JavaScript If Else Statements. if-else Statement in JavaScript. JavaScript's conditional statements: if; if-else; nested-if; if-else-if. These statements allow you to control the flow of your program's execution based upon conditions known only during run time.

Decision Making in programming is similar to decision making in real life. In programming also we face some situations where we want a certain block of code to be executed when some condition is fulfilled.
A programming language uses control statements to control the flow of execution of the program based on certain conditions. These are used to cause the flow of execution to advance and branch based on changes to the state of a program.

JavaScript’s conditional statements:

  • if
  • if-else
  • nested-if
  • if-else-if

These statements allow you to control the flow of your program’s execution based upon conditions known only during run time.

  • if: if statement is the most simple decision making statement. It is used to decide whether a certain statement or block of statements will be executed or not i.e if a certain condition is true then a block of statement is executed otherwise not.
    Syntax:
if(condition) 
{
   // Statements to execute if
   // condition is true
}

Here, condition after evaluation will be either true or false. if statement accepts boolean values – if the value is true then it will execute the block of statements under it.
If we do not provide the curly braces ‘{‘ and ‘}’ after if( condition ) then by default if statement will consider the immediate one statement to be inside its block. For example,

if(condition)
   statement1;
   statement2;

// Here if the condition is true, if block 
// will consider only statement1 to be inside 
// its block.

Flow chart:

Example:

<script type = "text/javaScript"> 

// JavaScript program to illustrate If statement 

var i = 10; 

if (i > 15) 
document.write("10 is less than 15"); 

// This statement will be executed 
// as if considers one statement by default 
document.write("I am Not in if"); 

< /script> 

Output:

I am Not in if
  • if-else: The if statement alone tells us that if a condition is true it will execute a block of statements and if the condition is false it won’t. But what if we want to do something else if the condition is false. Here comes the else statement. We can use the else statement with if statement to execute a block of code when the condition is false.
    Syntax:
if (condition)
{
    // Executes this block if
    // condition is true
}
else
{
    // Executes this block if
    // condition is false
}


Example:

<script type = "text/javaScript"> 

// JavaScript program to illustrate If-else statement 

var i = 10; 

if (i < 15) 
document.write("10 is less than 15"); 
else
document.write("I am Not in if"); 

< /script> 

Output:

i is smaller than 15
  • nested-if A nested if is an if statement that is the target of another if or else. Nested if statements means an if statement inside an if statement. Yes, JavaScript allows us to nest if statements within if statements. i.e, we can place an if statement inside another if statement.
    Syntax:
if (condition1) 
{
   // Executes when condition1 is true
   if (condition2) 
   {
      // Executes when condition2 is true
   }
}

Example:

<script type = "text/javaScript"> 

// JavaScript program to illustrate nested-if statement 

var i = 10; 

if (i == 10) { 

// First if statement 
if (i < 15) 
	document.write("i is smaller than 15"); 

// Nested - if statement 
// Will only be executed if statement above 
// it is true 
if (i < 12) 
	document.write("i is smaller than 12 too"); 
else
	document.write("i is greater than 15"); 
} 
< /script> 

Output:

i is smaller than 15
i is smaller than 12 too
  • if-else-if ladder Here, a user can decide among multiple options.The if statements are executed from the top down. As soon as one of the conditions controlling the if is true, the statement associated with that if is executed, and the rest of the ladder is bypassed. If none of the conditions is true, then the final else statement will be executed.
if (condition)
    statement;
else if (condition)
    statement;
.
.
else
    statement;


Example:

<script type = "text/javaScript"> 
// JavaScript program to illustrate nested-if statement 

var i = 20; 

if (i == 10) 
document.wrte("i is 10"); 
else if (i == 15) 
document.wrte("i is 15"); 
else if (i == 20) 
document.wrte("i is 20"); 
else
document.wrte("i is not present"); 
< /script> 

Output:

i is 20