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verhead 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 defined using either the var 69 let 70 or const 71 keywords variables defined without keywords will be defined at the global scope declares a function scoped variable named x and implicitly assigns the special value undefined to it variables without value are automatically set to undefined var is generally considered bad practice and let and const are usually preferred var x variables can be manually set to undefined like so let x2 undefined declares a block scoped variable named y and implicitly sets it to undefined the let keyword was introduced in ecmascript 2015 let y declares a block scoped un reassignable variable named z and sets it to a string literal the const keyword was also introduced in ecmascript 2015 and must be explicitly assigned to the keyword const means constant hence the variable cannot be reassigned as the value is constant const z this value cannot be reassigned declares a global scoped variable and assigns 3 this is generally considered bad practice and will not work if strict mode is on t 3 declares a variable named mynumber and assigns a number literal the value 2 to it let mynumber 2 reassigns mynumber setting it to a string literal the value foo javascript is a dynamically typed language so this is legal mynumber foo note the comments in the example above all of which were preceded with two forward slashes there is no built in input output functionality in javascript instead it is provided by the run time environment the ecmascript specification in edition 5 1 mentions that there are no provisions in this specification for input of external data or output of computed results 72 however most runtime environments have a console object that can be used to print output 73 here is a minimalist hello world program in javascript in a runtime environment with a console object console log hello world in html documents a program like this is required for an output text nodes can be made using the write method this is frowned upon as it can overwrite the document if the document is fully loaded document write foo elements can be made too first they have to be created in the dom const myelem document createelement span attributes like classes and the id can be set as well myelem classlist add foo myelem id bar after setting this the tag will look like this span class foo id bar data attr baz span myelem setattribute data attr baz which could also be written as myelem dataset attr baz finally append it as a child element to the body in the html document body appendchild myelem elements can be imperatively grabbed with queryselector for one element or queryselectorall for multiple elements that can be looped with foreach document queryselector class selects the first element with the class class document queryselector id selects the first element with an id of id document queryselector data other selects the first element with the data other attribute document queryselectorall multiple returns an array of all elements with the multiple class a simple recursive function to calculate the factorial of a natural number function factorial n checking the argument for legitimacy factorial is defined for positive integers if isnan n console error non numerical argument not allowed return nan the special value not a number if n 0 return 1 0 1 if n 0 return undefined factorial of negative numbers is not defined if n 1 console warn n will be rounded to the closest integer for non integers consider using gamma function instead n math round n the above checks need not be repeated in the recursion hence defining the actual recursive part separately below the following line is a function expression to recursively compute the factorial it uses the arrow syntax introduced in es6 const recursivelycompute a a 1 a recursivelycompute a 1 1 note the use of the ternary operator return recursivelycompute n factorial 3 returns 6 an anonymous function or lambda const counter function let count 0 return function return count const x counter x returns 1 x returns 2 x returns 3 this example shows that in javascript function closures capture their non local variables by reference arrow functions were first introduced in 6th edition ecmascript 2015 they shorten the syntax for writing functions in javascript arrow functions are anonymous so a variable is needed to refer to them in order to invoke them after their creation unless surrounded by parenthesis and executed immediately example of arrow function arrow functions let us omit the function keyword here long_example points to an anonymous function value const long_example input1 input2 console log hello world const output input1 input2 return output if there are no braces the arrow function simply returns the expression so here it s input1 input2 const short_example input1 input2 input1 input2 long_example 2 3 prints hello world and returns 5 short_example 2 5 returns 7 if an arrow function has only one parameter the parentheses can be removed const no_parentheses input input 2 no_parentheses 3 returns 5 an arrow function like other function definitions can be executed in the same statement as they are created this is useful when writing libraries to avoid filling the global scope and for closures let three a b a b 1 2 const generate_multiplier_function a b isnan b b a a b const five_multiples generate_multiplier_function 5 the supplied argument seeds the expression and is retained by a five_multiples 1 returns 5 five_multiples 3 returns 15 five_multiples 4 returns 60 in javascript objects can be created as instances of a class object class example class ball constructor radius this radius radius this area math pi radius 2 classes and thus objects can contain functions known as methods show console log this radius const myball new ball 5 creates a new instance of the ball object with radius 5 myball radius object properties can usually be modified from the outside myball show using the inherited show function logs 6 in javascript objects can be instantiated directly from a function object functional example function ball radius const area math pi radius 2 const obj radius area objects are mutable and functions can be added as properties obj show console log obj radius return obj const myball ball 5 creates a new ball object with radius 5 no new keyword needed myball radius the instance property can be modified myball show using the show function logs 6 the new instance value variadic function demonstration arguments is a special variable 74 function sum let x 0 for let i 0 i arguments length i x arguments i return x sum 1 2 returns 3 sum 1 2 3 returns 6 as of es6 using the rest operator function sum args return args reduce a b a b sum 1 2 returns 3 sum 1 2 3 returns 6 immediately invoked function expressions are often used to create closures closures allow gathering properties and methods in a namespace and making some of them private let counter function let i 0 private property return public methods get function alert i set function value i value increment function alert i module counter get returns 0 counter set 6 counter increment returns 7 counter increment returns 8 generator objects in the form of generator functions provide a function which can be called exited and re entered while maintaining internal context statefulness 75 function rawcounter yield 1 yield 2 function dynamiccounter let count 0 while true it is not recommended to utilize while true loops in most cases yield count instances const counter1 rawcounter const counter2 dynamiccounter implementation counter1 next value 1 done false counter1 next value 2 done false counter1 next value undefined done true counter2 next value 1 done false counter2 next value 2 done false counter2 next value 3 done false infinitely javascript can export and import from modules 76 export example mymodule js this function remains private as it is not exported let sum a b return a b export variables export let name alice export let age 23 export named functions export function add num1 num2 return num1 num2 export class export class multiplication constructor num1 num2 this num1 num1 this num2 num2 add return sum this num1 this num2 import example import one property import add from mymodule js console log add 1 2 3 import multiple properties import name age from mymodule js console log name age alice 23 import all properties from a module import from module js console log name age alice 23 console log add 1 2 3 more advanced example this sample code displays various javascript features finds the lowest common multiple lcm of two numbers function lcmcalculator x y constructor function if isnan x y throw new typeerror non numeric arguments not allowed const checkint function x inner function if x 1 0 throw new typeerror x is not an integer return x this a checkint x semicolons are optional a newline is enough this b checkint y the prototype of object instances created by a constructor is that constructor s prototype property lcmcalculator prototype object literal constructor lcmcalculator when reassigning a prototype set the constructor property appropriately gcd function method that calculates the greatest common divisor euclidean algorithm let a math abs this a b math abs this b t if a b swap variables t b b a a t a b b a swap using destructuring assignment es6 while b 0 t b b a b a t only need to calculate gcd once so redefine this method actually not redefinition it s defined on the instance itself so that this gcd refers to this redefinition instead of lcmcalculator prototype gcd note that this leads to a wrong result if the lcmcalculator object members a and or b are altered afterwards also gcd gcd this gcd this gcd this gcd function return a return a object property names can be specified by strings delimited by double or single quotes lcm function variable names do not collide with object properties e g lcm is not this lcm not using this a this b to avoid fp precision issues let lcm this a this gcd this b only need to calculate lcm once so redefine this method this lcm function return lcm return lcm methods can also be declared using es6 syntax tostring using both es6 template literals and the operator to concatenate values return lcmcalculator a this a b this b define generic output function this implementation only works for web browsers function output x document body appendchild document createtextnode x document body appendchild document createelement br note array s map and foreach are defined in javascript 1 6 they are used here to demonstrate javascript s inherent functional nature 25 55 21 56 22 58 28 56 map function pair array literal mapping function return new lcmcalculator pair 0 pair 1 sort a b a lcm b lcm sort with this comparative function is a shorthand form of a function called arrow function foreach printresult function printresult obj output obj gcd obj gcd lcm obj lcm the following output should be displayed in the browser window lcmcalculator a 28 b 56 gcd 28 lcm 56 lcmcalculator a 21 b 56 gcd 7 lcm 168 lcmcalculator a 25 b 55 gcd 5 lcm 275 lcmcalculator a 22 b 58 gcd 2 lcm 638 security see also browser security javascript and the dom provide the potential for malicious authors to deliver scripts to run on a client computer via the web browser authors minimize this risk using two restrictions first scripts run in a sandbox in which they can only perform web related actions not general purpose programming tasks like creating files second scripts are constrained by the same origin policy scripts from one web site do not have access to information such as usernames passwords or cookies sent to another site most javascript related security bugs are breaches of either the same origin policy or the sandbox there are subsets of general javascript adsafe secure ecmascript ses that provide greater levels of security especially on code created by third parties such as advertisements 77 78 closure toolkit is another project for safe embedding and isolation of third party javascript and html 79 content security policy is the main intended method of ensuring that only trusted code is executed on a web page cross site vulnerabilities main articles cross site scripting and cross site request forgery a common javascript related security problem is cross site scripting xss a violation of the same origin policy xss vulnerabilities occur when an attacker can cause a target web site such as an online banking website to include a malicious script in the webpage presented to a victim the script in this example can then access the banking application with the privileges of the victim potentially disclosing secret information or transferring money without the victim s authorization a solution to xss vulnerabilities is to use html escaping whenever displaying untrusted data some browsers include partial protection against reflected xss attacks in which the attacker provides a url including malicious script however even users of those browsers are vulnerable to other xss attacks such as those where the malicious code is stored in a database only correct design of web applications on the server side can fully prevent xss xss vulnerabilities can also occur because of implementation mistakes by browser authors 80 another cross site vulnerability is cross site request forgery csrf in csrf code on an attacker s site tricks the victim s browser into taking actions the user did not intend at a target site like transferring money at a bank when target sites rely solely on cookies for request authentication requests originating from code on the attacker s site can carry the same valid login credentials of the initiating user in general the solution to csrf is to require an authentication value in a hidden form field and not only in the cookies to authenticate any request that might have lasting effects checking the http referrer header can also help javascript hijacking is a type of csrf attack in which a script tag on an attacker s site exploits a page on the victim s site that returns private information such as json or javascript possible solutions include requiring an authentication token in the post and get parameters for any response that returns private information misplaced trust in the client developers of client server applications must recognize that untrusted clients may be under the control of attackers the application author cannot assume that their javascript c...