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at formed around them many new libraries were created including jquery prototype dojo toolkit and mootools google debuted its chrome browser in 2008 with the v8 javascript engine that was faster than its competition 22 23 the key innovation was just in time compilation jit 24 so other browser vendors needed to overhaul their engines for jit 25 in july 2008 these disparate parties came together for a conference in oslo this led to the eventual agreement in early 2009 to combine all relevant work and drive the language forward the result was the ecmascript 5 standard released in december 2009 reaching maturity ambitious work on the language continued for several years culminating in an extensive collection of additions and refinements being formalized with the publication of ecmascript 6 in 2015 26 the creation of node js in 2009 by ryan dahl sparked a significant increase in the usage of javascript outside of web browsers node combines the v8 engine an event loop and i o apis thereby providing a stand alone javascript runtime system 27 28 as of 2018 node had been used by millions of developers 29 and npm had the most modules of any package manager in the world 30 the ecmascript draft specification is currently maintained openly on github and editions are produced via regular annual snapshots 31 potential revisions to the language are vetted through a comprehensive proposal process 32 33 now instead of edition numbers developers check the status of upcoming features individually 31 the current javascript ecosystem has many libraries and frameworks established programming practices and substantial usage of javascript outside of web browsers plus with the rise of single page applications and other javascript heavy websites several transpilers have been created to aid the development process 34 trademark javascript is a trademark of oracle corporation in the united states 35 36 the trademark was originally issued to sun microsystems on 6 may 1997 and was transferred to oracle when they acquired sun in 2010 website client side usage javascript is the dominant client side scripting language of the web with 98 of all websites mid 2022 using it for this purpose 37 scripts are embedded in or included from html documents and interact with the dom all major web browsers have a built in javascript engine that executes the code on the user s device examples of scripted behavior loading new web page content without reloading the page via ajax or a websocket for example users of social media can send and receive messages without leaving the current page web page animations such as fading objects in and out resizing and moving them playing browser games controlling the playback of streaming media generating pop up ads or alert boxes validating input values of a web form before the data is sent to a web server logging data about the user s behavior then sending it to a server the website owner can use this data for analytics ad tracking and personalization redirecting a user to another page storing and retrieving data on the user s device via the storage or indexeddb standards libraries and frameworks over 80 of websites use a third party javascript library or web framework for their client side scripting 38 jquery is by far the most popular library used by over 75 of websites 38 facebook created the react library for its website and later released it as open source other sites including twitter now use it likewise the angular framework created by google for its websites including youtube and gmail is now an open source project used by others 38 in contrast the term vanilla js has been coined for websites not using any libraries or frameworks instead relying entirely on standard javascript functionality 39 other usage the use of javascript has expanded beyond its web browser roots javascript engines are now embedded in a variety of other software systems both for server side website deployments and non browser applications initial attempts at promoting server side javascript usage were netscape enterprise server and microsoft s internet information services 40 41 but they were small niches 42 server side usage eventually started to grow in the late 2000s with the creation of node js and other approaches 42 electron cordova react native and other application frameworks have been used to create many applications with behavior implemented in javascript other non browser applications include adobe acrobat support for scripting pdf documents 43 and gnome shell extensions written in javascript 44 javascript has recently begun to appear in some embedded systems usually by leveraging node js 45 46 47 features the following features are common to all conforming ecmascript implementations unless explicitly specified otherwise imperative and structured javascript supports much of the structured programming syntax from c e g if statements while loops switch statements do while loops etc one partial exception is scoping originally javascript only had function scoping with var block scoping was added in ecmascript 2015 with the keywords let and const like c javascript makes a distinction between expressions and statements one syntactic difference from c is automatic semicolon insertion which allow semicolons which terminate statements to be omitted 48 weakly typed javascript is weakly typed which means certain types are implicitly cast depending on the operation used 49 the binary operator casts both operands to a string unless both operands are numbers this is because the addition operator doubles as a concatenation operator the binary operator always casts both operands to a number both unary operators always cast the operand to a number values are cast to strings like the following 49 strings are left as is numbers are converted to their string representation arrays have their elements cast to strings after which they are joined by commas other objects are converted to the string object object where object is the name of the constructor of the object values are cast to numbers by casting to strings and then casting the strings to numbers these processes can be modified by defining tostring and valueof functions on the prototype for string and number casting respectively javascript has received criticism for the way it implements these conversions as the complexity of the rules can be mistaken for inconsistency 50 49 for example when adding a number to a string the number will be cast to a string before performing concatenation but when subtracting a number from a string the string is cast to a number before performing subtraction javascript type conversions left operand operator right operand result empty array empty array empty string empty array empty object object object string false boolean empty array false string 123 string 1 number 1231 string 123 string 1 number 122 number 123 string abc string nan number often also mentioned is resulting in 0 number this is misleading the is interpreted as an empty code block instead of an empty object and the empty array is cast to a number by the remaining unary operator if you wrap the expression in parentheses the curly brackets are interpreted as an empty object and the result of the expression is object object as expected 49 dynamic typing javascript is dynamically typed like most other scripting languages a type is associated with a value rather than an expression for example a variable initially bound to a number may be reassigned to a string 51 javascript supports various ways to test the type of objects including duck typing 52 run time evaluation javascript includes an eval function that can execute statements provided as strings at run time object orientation prototype based prototypal inheritance in javascript is described by douglas crockford as you make prototype objects and then make new instances objects are mutable in javascript so we can augment the new instances giving them new fields and methods these can then act as prototypes for even newer objects we don t need classes to make lots of similar objects objects inherit from objects what could be more object oriented than that 53 in javascript an object is an associative array augmented with a prototype see below each key provides the name for an object property and there are two syntactical ways to specify such a name dot notation obj x 10 and bracket notation obj x 10 a property may be added rebound or deleted at run time most properties of an object and any property that belongs to an object s prototype inheritance chain can be enumerated using a for in loop prototypes javascript uses prototypes where many other object oriented languages use classes for inheritance 54 it is possible to simulate many class based features with prototypes in javascript 55 functions as object constructors functions double as object constructors along with their typical role prefixing a function call with new will create an instance of a prototype inheriting properties and methods from the constructor including properties from the object prototype 56 ecmascript 5 offers the object create method allowing explicit creation of an instance without automatically inheriting from the object prototype older environments can assign the prototype to null 57 the constructor s prototype property determines the object used for the new object s internal prototype new methods can be added by modifying the prototype of the function used as a constructor javascript s built in constructors such as array or object also have prototypes that can be modified while it is possible to modify the object prototype it is generally considered bad practice because most objects in javascript will inherit methods and properties from the object prototype and they may not expect the prototype to be modified 58 functions as methods unlike many object oriented languages there is no distinction between a function definition and a method definition rather the distinction occurs during function calling when a function is called as a method of an object the function s local this keyword is bound to that object for that invocation functional javascript functions are first class a function is considered to be an object 59 as such a function may have properties and methods such as call and bind 60 a nested function is a function defined within another function it is created each time the outer function is invoked in addition each nested function forms a lexical closure the lexical scope of the outer function including any constant local variable or argument value becomes part of the internal state of each inner function object even after execution of the outer function concludes 61 javascript also supports anonymous functions delegative javascript supports implicit and explicit delegation functions as roles traits and mixins javascript natively supports various function based implementations of role 62 patterns like traits 63 64 and mixins 65 such a function defines additional behavior by at least one method bound to the this keyword within its function body a role then has to be delegated explicitly via call or apply to objects that need to feature additional behavior that is not shared via the prototype chain object composition and inheritance whereas explicit function based delegation does cover composition in javascript implicit delegation already happens every time the prototype chain is walked in order to e g find a method that might be related to but is not directly owned by an object once the method is found it gets called within this object s context thus inheritance in javascript is covered by a delegation automatism that is bound to the prototype property of constructor functions miscellaneous javascript is a zero index language run time environment javascript typically relies on a run time environment e g a web browser to provide objects and methods by which scripts can interact with the environment e g a web page dom these environments are single threaded javascript also relies on the run time environment to provide the ability to include import scripts e g html script elements this is not a language feature per se but it is common in most javascript implementations javascript processes messages from a queue one at a time javascript calls a function associated with each new message creating a call stack frame with the function s arguments and local variables the call stack shrinks and grows based on the function s needs when the call stack is empty upon function completion javascript proceeds to the next message in the queue this is called the event loop described as run to completion because each message is fully processed before the next message is considered however the language s concurrency model describes the event loop as non blocking program input output is performed using events and callback functions this means for instance that javascript can process a mouse click while waiting for a database query to return information 66 variadic functions an indefinite number of parameters can be passed to a function the function can access them through formal parameters and also through the local arguments object variadic functions can also be created by using the bind method array and object literals like many scripting languages arrays and objects associative arrays in other languages can each be created with a succinct shortcut syntax in fact these literals form the basis of the json data format regular expressions javascript also supports regular expressions in a manner similar to perl which provide a concise and powerful syntax for text manipulation that is more sophisticated than the built in string functions 67 promises and async await javascript supports promises and async await for handling asynchronous operations a built in promise object provides functionality for handling promises and associating handlers with an asynchronous action s eventual result recently combinator methods were introduced in the javascript specification which allows developers to combine multiple javascript promises and do operations based on different scenarios the methods introduced are promise race promise all promise allsettled and promise any async await allows an asynchronous non blocking function to be structured in a way similar to an ordinary synchronous function asynchronous non blocking code can be written with minimal overhead structured similar to traditional synchronous blocking code vendor specific extensions historically some javascript engines supported these non standard features conditional catch clauses like java array comprehensions and generator expressions like python concise function expressions function args expr this experimental syntax predated arrow functions ecmascript for xml e4x an extension that adds native xml support to ecmascript unsupported in firefox since version 21 68 syntax main article javascript syntax simple examples variables in javascript can be defi...
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