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    Introduction

    Welcome!

    Welcome! Overview

    Welcome to Introduction to JavaScript! This class is a lab-based, hands-on, problem-solving class that will turn you into a JavaScript adept.

    During this course, you will:

    Develop familiarity with the JavaScript language.

    Learn to use best-practice idioms and patterns.

    Understand concepts commonly used in dynamic language programming, such as introspection, higher-order functions, and closures.

    Understand advanced JavaScript features such as prototypical inheritence.

    Become familiar with common JavaScript libraries and tools that are used in web application development.

    Are you in the right class? You need to have taken one of CS106A or CS106B. You should be comfortable coding in at least one other programming language. This class is not intended for fist-time programmers: a more appropriate class would be CS105 or CS106A. Understanding of HTML and CSS is helpful but not necessary, as we will review HTML and CSS later. If you have any previous experience developing web applications, the first part of the course may be too basic for you, but if you stick around, we will cover some advanced JavaScript language concepts, as well as some useful libraries.

    What is JavaScript?

    What is JavaScript? JavaScript is primarily a client-side language. JavaScript started at Netscape, a web browser developed in the 1990s. A webpage can contain embedded JavaScript, which executes when a user visits the page. The language was created to allow web developers to embed executable code on their webpages, so that they could make their webpages interactive, or perform simple tasks. Today, browser scripting remains the main use-case of JavaScript.

    JavaScript’s syntax is heavily inspired by C++ and Java. If you have experience in C++ or Java, JavaScript’s syntax will seem familiar to you. However, the inner workings of JavaScript is closer to a dynamically-typed, interpreted language such as Python or Ruby.

    JavaScript is an interpreted language, not a compiled language. A program such as C++ or Java needs to be compiled before it is run. The source code is passed through a program called a compiler, which translates it into bytecode that the machine understands and can execute. In contrast, JavaScript has no compilation step. Instead, an interpreter in the browser reads over the JavaScript code, interprets each line, and runs it. More modern browsers use a technology known as Just-In-Time (JIT) compilation, which compiles JavaScript to executable bytecode just as it is about to run.

    JavaScript is named after Java, and many ideas are borrowed from the Java language. Other than that, Java and JavaScript are two entirely distinct languages. The most significant difference between them is that Java is a compiled language, and JavaScript is a interpreted language. JavaScript runs on many browsers out-of-the-box, whereas Java applets require an additional plug in. Both languages have different runtime environments, different governing bodies, different libraries.

    Despite all its faults, JavaScript is a very useful language. JavaScript runs in every web browser, out of the box. A JavaScript application runs on every device, whereas a desktop or mobile application runs only on the application it is targeted to (Windows, Mac OSX, Linux, iPhone, Android). This allows you to write cross-platform apps in a really easy way. JavaScript’s role has also expanded significantly. Platforms such as Node.js allow developers to run JavaScript server-side. It is now possible to create entire web applications in which both client-side and server-side logic is written in JavaScript.

    JavaScript can be a very complicated language, and most teams use only a subset of JavaScript. If you read a style guide, it will recommend the use of particular JavaScript techniques, constructs and libraries. Since JavaScript is so messy, this class will make a number of recommendations on what we consider good style. However, we will cover a wide range of styles so that you can jump into a new team and pick up the style quickly.

    Live Code Examples

    Live code examples are scattered throughout this text. These snippets of code can be edited and run from within the browser. Click and type within the box to edit, and press the “Run” button to see what they do.

    Hello World!

    Brian Kernighan and Dennis Ritchie taught that the first thing you should do when learning a new programming language is learn out to print the words “Hello, World!” Here is the program in JavaScript:

    console.log("Hello world!");

    The console.log function (or method) prints its argument to the console, followed by a newline. We will be using it a lot.

    Useful functions

    There are two other useful functions that you might see often. The alert function displays a text box with the specified message.

    alert("Hello world!");

    The prompt function displays a box prompting the user for input.

    alert(prompt("What is your name?"));

    View Source

    JavaScript is everywhere on the web. The interactive elements on this page were implemented in JavaScript, of course.

    There are two main ways to put JavaScript on the web page, and they both involve the . Note that in this case, the contents of the script tag should be empty.

    स्रोत : web.stanford.edu

    How to run JavaScript Programs (with examples) and How it works?

    This article covers How does JavaScript run in a web-browser? How to embed and execute and diagnose an error in JavaScript in a web-browser?

    How to run JavaScript Programs (with examples)?

    Category: JavaScript , September 29 2021

    0 0 8 min read Author Arunkumar Chandra Reviewers Ravinder Singh Previous Lesson What is JavaScript? Next Lesson

    JavaScript Variables

    Being a client-side scripting language, JavaScript needs some clients to interpret and execute it. In the case of web applications, we can use any of the supported web browsers as a platform for executing JavaScript. In the below sections, let's understand how does JavaScript run in a web browser and how the JavaScript engine interprets the JavaScript code. Moreover, we will also understand how JavaScript can integrate under various sections of an HTML page.

    How does JavaScript run in a web-browser?

    How to embed and execute JavaScript in a web-browser?

    Procedure to embed and execute JavaScript in the HTML tag?

    How to embed and execute JavaScript in HTML ?

    How to embed an external JavaScript file in HTML?

    Procedure to diagnose an error in JavaScript?

    How does JavaScript run in a web-browser?

    JavaScript is an interpreted language where code is explained and executed at the runtime. Additionally, we know that web browsers understand HTML and CSS and converting those languages into a visual display on the screen. The part of the web browser that understands HTML and CSS is called the rendering engine. However, most browsers also have a JavaScript interpreter. That’s the part of the browser that understands JavaScript and run JavaScript programs. A typical architecture of a browser looks like below:

    As we can see, the browser's Rendering engine interacts with the "JavaScript's Interpreter" and sends the JavaScript code to the JavaScript engine for processing.

    The JavaScript engine/JavaScript Interpreter comprises both an Interpreter and a Just in Time compiler. Additionally, the overall execution of the JavaScript is a 5 step process, as shown in the below image:

    JavaScript Engine Loads Source Code.

    The interpreter starts the application.

    The Compiler receives code.

    The compiler starts optimization and compilation.

    The compiler incrementally optimizes the code.

    Now when we talk about JavaScript engines, each of the browsers have their pre-installed engines. Below are few of the JavaScript engines developed and used by some major browsers:

    V8 Engine: This engine is developed by Google, which is one of the most used Engines in chrome, chromium, and even in NodeJS.Spider Monkey: Mozilla developed this engine used in the Firefox browser.JavaScriptCore: This engine s developed by Apple used in the Safari browser.Chakra: Microsoft developed this engine used in the Edge browser and internet explorer.Hermes Engine: Facebook developed this engine used in android apps.

    How to embed and run JavaScript in a web-browser?

    The web browser is usually expecting HTML, so you must specifically tell the browser when JavaScript is coming by using the tag, it knows it’s reached the end of the JavaScript program and can get back to its normal duties. Let's understand in following sections, how the JavaScript code embeds inside an HTML file:

    How to embed and execute JavaScript in the HTML tag?

    The

    Open any text editor like notepad++ and type the above code. Save the file with name  helloworld.html and open it in any browser (Chrome, Firefox, or IE). It should show the output as:

    As seen in the above example, "document.write" is a function used to write data into the Html page. When we open an Html file in the browser, the JavaScript present under script tag executes. We will discuss more details above other functionalities, like write in the DOM Manipulation article.

    How to embed and run JavaScript in HTML tag?

    स्रोत : www.toolsqa.com

    JavaScript Execution Context – How JS Works Behind The Scenes

    All JavaScript code needs to be hosted and run in some kind of environment. In most cases, that environment would be a web browser. For any piece of JavaScript code to be executed in a web browser, a lot of processes take place behind the scenes. In this article, we'll

    FEBRUARY 10, 2022 / #JAVASCRIPT

    JavaScript Execution Context – How JS Works Behind The Scenes

    Victor Ikechukwu

    All JavaScript code needs to be hosted and run in some kind of environment. In most cases, that environment would be a web browser.

    For any piece of JavaScript code to be executed in a web browser, a lot of processes take place behind the scenes. In this article, we'll take a look at everything that happens behind the scenes for JavaScript code to run in a web browser.

    Before we dive in, here are some prerequisites to familiarize yourself with, because we'll use them often in this article.

    Parser: A Parser or Syntax Parser is a program that reads your code line-by-line. It understands how the code fits the syntax defined by the Programming Language and what it (the code) is expected to do.JavaScript Engine: A JavaScript engine is simply a computer program that receives JavaScript source code and compiles it to the binary instructions (machine code) that a CPU can understand. JavaScript engines are typically developed by web browser vendors, and each major browser has one. Examples include the V8 engine for Google chrome, SpiderMonkey for Firefox, and Chakra for Internet Explorer.Function Declarations: These are functions that are assigned a name.

    function doSomething() { //here "doSomething" is the function's name

    statements; }

    Function Expressions: These are anonymous functions, that is functions without a function name like js function () { statements }. They are usually used in statements, like assigning a function to a variable. let someValue = function () { statements }.

    Now, that we've gotten those out of the way, let's dive in.

    How JavaScript Code Gets Executed

    For does who don't know, the browser doesn't natively understand the high-level JavaScript code that we write in our applications. It needs to be converted into a format that the browser and our computers can understand – machine code.

    While reading through HTML, if the browser encounters JavaScript code to run via a

    स्रोत : www.freecodecamp.org

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