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bject 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 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 semicolon...
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