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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 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 inpu...
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