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JavaScript har primitive, objekter og funktioner. Alle er værdier. Alle behandles som genstande, selv primitiver.

Primitiver

Nummer, boolsk, streng undefinedog nuller primitiver.

Nummer

Der er kun én nummertype i JavaScript, den 64-bit binære flydende punkt-type. Decimaltalets aritmetik er unøjagtig.

Som du måske allerede ved, 0.1 + 0.2gør det ikke 0.3. Men med heltal er aritmetikken nøjagtig, så 1+2 === 3.

Tal arver metoder fra Number.prototypeobjektet. Metoder kan kaldes på numre:

(123).toString(); //"123" (1.23).toFixed(1); //"1.2"

Der er funktioner til konvertering strenge til tal: Number.parseInt(), Number.parseFloat()og Number():

Number.parseInt("1") //1 Number.parseInt("text") //NaN Number.parseFloat("1.234") //1.234 Number("1") //1 Number("1.234") //1.234

Ugyldige aritmetiske operationer eller ugyldige konverteringer kaster ikke en undtagelse, men vil resultere i NaNværdien "Ikke-et-nummer". Number.isNaN()kan registrere NaN.

Den +Operatøren kan tilføje eller SAMMENKÆDNING.

1 + 1 //2 "1" + "1" //"11" 1 + "1" //"11"

Snor

En streng gemmer en række Unicode-tegn. Teksten kan være inde i dobbelt anførselstegn ""eller enkelt anførselstegn ''.

Strenge arver metoder fra String.prototype. De har metoder som: substring(), indexOf()og concat().

"text".substring(1,3) //"ex" "text".indexOf('x') //2 "text".concat(" end") //"text end"

Strenge, som alle primitiver, er uforanderlige. For eksempel concat()ændrer ikke den eksisterende streng, men opretter en ny.

Boolsk

En boolsk har to værdier: trueog false.

Sproget har sandfærdige og falske værdier.

false, null, undefined, ''(Tom streng), 0 og NaNer falsy. Alle andre værdier, inklusive alle objekter, er sandfærdige.

Sandhedsværdien vurderes til, truenår den udføres i en boolsk sammenhæng. Falsy-værdi evalueres til false. Se på det næste eksempel, der viser falsegrenen.

let text = ''; if(text) { console.log("This is true"); } else { console.log("This is false"); }

Ligestillingsoperatøren er ===. Den ikke lige operatør er !==.

Variabler

Variabler kan defineres ved hjælp af var, letog const.

varerklærer og valgfrit initialiserer en variabel. Variabler, der er deklareret med, varhar et funktionsomfang. De behandles som erklæret øverst i funktionen. Dette kaldes variabel hejsning.

Den leterklæring har en blok rækkevidde.

Værdien af ​​en variabel, der ikke initialiseres, er undefined.

En variabel, der er deklareret med, constkan ikke tildeles igen. Dens værdi kan dog stadig ændres. constfryser variablen, Object.freeze()fryser objektet. Den consterklæring har en blok rækkevidde.

Objekter

Et objekt er en dynamisk samling af egenskaber.

Ejendomsnøglen er en unik streng. Når en ikke-streng bruges som egenskabsnøglen, konverteres den til en streng. Ejendomsværdien kan være en primitiv, objekt eller funktion.

Den enkleste måde at oprette et objekt på er at bruge et bogstaveligt objekt:

let obj = { message : "A message", doSomething : function() {} }

Der er to måder at få adgang til egenskaber: priknotation og parentesnotation. Vi kan til enhver tid læse, tilføje, redigere og fjerne et objekts egenskaber.

  • får: object.name,object[expression]
  • sæt: object.name = value,object[expression] = value
  • slette: delete object.name,delete object[expression]
let obj = {}; //create empty object obj.message = "A message"; //add property obj.message = "A new message"; //edit property delete obj.message; //delete property

Objekter kan bruges som kort. Et simpelt kort kan oprettes ved hjælp af Object.create(null):

let french = Object.create(null); french["yes"] = "oui"; french["no"] = "non"; french["yes"];//"oui"

Alle objekts ejendomme er offentlige. Object.keys()kan bruges til at gentage alle egenskaber.

function logProperty(name){ console.log(name); //property name console.log(obj[name]); //property value } Object.keys(obj).forEach(logProperty);

Object.assign()kopierer alle egenskaber fra et objekt til et andet. Et objekt kan klones ved at kopiere alle dets egenskaber til et tomt objekt:

let book = { title: "The good parts" }; let clone = Object.assign({}, book);

Et uforanderligt objekt er et objekt, som en gang er oprettet, ikke kan ændres. Hvis du vil gøre objektet uforanderligt, skal du bruge det Object.freeze().

Primitiver vs objekter

Primitiver (undtagen nullog undefined) behandles som objekter i den forstand, at de har metoder, men de er ikke objekter.

Tal, strenge og booleans har indpakningsækvivalenter. Det er de Number, Stringog Booleanfunktioner.

In order to allow access to properties on primitives, JavaScript creates an wrapper object and then destroys it. The process of creating and destroying wrapper objects is optimized by the JavaScript engine.

Primitives are immutable, and objects are mutable.

Array

Arrays are indexed collections of values. Each value is an element. Elements are ordered and accessed by their index number.

JavaScript has array-like objects. Arrays are implemented using objects. Indexes are converted to strings and used as names for retrieving values.

A simple array like let arr = ['A', 'B', 'C'] is emulated using an object like the one below:

{ '0': 'A', '1': 'B', '2': 'C' }

Note that arr[1] gives the same value as arr['1'] : arr[1] === arr['1'] .

Removing values from the array with delete will leave holes. splice() can be used to avoid the problem, but it can be slow.

let arr = ['A', 'B', 'C']; delete arr[1]; console.log(arr); // ['A', empty, 'C'] console.log(arr.length); // 3

JavaScript’s arrays don’t throw “index out of range” exceptions. If the index is not available, it will return undefined.

Stack and queue can easily be implemented using the array methods:

let stack = []; stack.push(1); // [1] stack.push(2); // [1, 2] let last = stack.pop(); // [1] console.log(last); // 2 let queue = []; queue.push(1); // [1] queue.push(2); // [1, 2] let first = queue.shift();//[2] console.log(first); // 1

Functions

Functions are independent units of behavior.

Functions are objects. Functions can be assigned to variables, stored in objects or arrays, passed as an argument to other functions, and returned from functions.

There are three ways to define a function:

  • Function Declaration (aka Function Statement)
  • Function Expression (aka Function Literal)
  • Arrow Function

The Function Declaration

  • function is the first keyword on the line
  • it must have a name
  • it can be used before definition. Function declarations are moved, or “hoisted”, to the top of their scope.
function doSomething(){}

The Function Expression

  • function is not the first keyword on the line
  • the name is optional. There can be an anonymous function expression or a named function expression.
  • it needs to be defined, then it can execute
  • it can auto-execute after definition (called “IIFE” Immediately Invoked Function Expression)
let doSomething = function() {}

Arrow Function

The arrow function is a sugar syntax for creating an anonymous functionexpression.

let doSomething = () => {};

Arrow functions don’t have their own this and arguments.

Function invocation

A function, defined with the function keyword, can be invoked in different ways:

  • Function form
doSomething(arguments)
  • Method form
theObject.doSomething(arguments) theObject["doSomething"](arguments)
  • Constructor form
new Constructor(arguments)
  • Apply form
 doSomething.apply(theObject, [arguments]) doSomething.call(theObject, arguments)

Functions can be invoked with more or fewer arguments than declared in the definition. The extra arguments will be ignored, and the missing parameters will be set to undefined.

Functions (except arrow functions) have two pseudo-parameters: this and arguments.

this

Methods are functions that are stored in objects. Functions are independent. In order for a function to know on which object to work onthis is used. this represents the function’s context.

There is no point to use this when a function is invoked with the function form: doSomething(). In this case this is undefined or is the window object, depending if the strict mode is enabled or not.

When a function is invoked with the method form theObject.doSomething(),this represents the object.

When a function is used as a constructor new Constructor(), thisrepresents the newly created object.

The value of this can be set with apply() or call():doSomething.apply(theObject). In this case this is the object sent as the first parameter to the method.

The value of this depends on how the function was invoked, not where the function was defined. This is of course a source of confusion.

arguments

The arguments pseudo-parameter gives all the arguments used at invocation. It’s an array-like object, but not an array. It lacks the array methods.

function log(message){ console.log(message); } function logAll(){ let args = Array.prototype.slice.call(arguments); return args.forEach(log); } logAll("msg1", "msg2", "msg3");

An alternative is the new rest parameters syntax. This time args is an array object.

function logAll(...args){ return args.forEach(log); }

return

A function with no return statement returns undefined. Pay attention to the automatic semi-colon insertion when using return. The following function will not return an empty object, but rather an undefined one.

function getObject(){ return { } } getObject()

To avoid the issue, use { on the same line as return :

function getObject(){ return { } }

Dynamic Typing

JavaScript has dynamic typing. Values have types, variables do not. Types can change at run time.

function log(value){ console.log(value); } log(1); log("text"); log({message : "text"});

The typeof() operator can check the type of a variable.

let n = 1; typeof(n); //number let s = "text"; typeof(s); //string let fn = function() {}; typeof(fn); //function

A Single Thread

The main JavaScript runtime is single threaded. Two functions can’t run at the same time. The runtime contains an Event Queue which stores a list of messages to be processed. There are no race conditions, no deadlocks.However, the code in the Event Queue needs to run fast. Otherwise the browser will become unresponsive and will ask to kill the task.

Exceptions

JavaScript has an exception handling mechanism. It works like you may expect, by wrapping the code using the try/catch statement. The statement has a single catch block that handles all exceptions.

It’s good to know that JavaScript sometimes has a preference for silent errors. The next code will not throw an exception when I try to modify a frozen object:

let obj = Object.freeze({}); obj.message = "text";

Strict mode eliminates some JavaScript silent errors. "use strict"; enables strict mode.

Prototype Patterns

Object.create(), constructor function, and class build objects over the prototype system.

Consider the next example:

let servicePrototype = { doSomething : function() {} } let service = Object.create(servicePrototype); console.log(service.__proto__ === servicePrototype); //true

Object.create() builds a new object service which has theservicePrototype object as its prototype. This means that doSomething() is available on the service object. It also means that the __proto__ property of service points to the servicePrototype object.

Let’s now build a similar object using class.

class Service { doSomething(){} } let service = new Service(); console.log(service.__proto__ === Service.prototype);

All methods defined in the Service class will be added to theService.prototype object. Instances of the Service class will have the same prototype (Service.prototype) object. All instances will delegate method calls to the Service.prototype object. Methods are defined once onService.prototype and then inherited by all instances.

Prototype chain

Objects inherit from other objects. Each object has a prototype and inherits their properties from it. The prototype is available through the “hidden” property __proto__ .

When you request a property which the object does not contain, JavaScript will look down the prototype chain until it either finds the requested property, or until it reaches the end of the chain.

Functional Patterns

JavaScript has first class functions and closures. These are concepts that open the way for Functional Programming in JavaScript. As a result, higher order functions are possible.

filter(), map(), reduce() are the basic toolbox for working with arrays in a function style.

filter()selects values from a list based on a predicate function that decides what values should be kept.

map() transforms a list of values to another list of values using a mapping function.

let numbers = [1,2,3,4,5,6]; function isEven(number){ return number % 2 === 0; } function doubleNumber(x){ return x*2; } let evenNumbers = numbers.filter(isEven); //2 4 6 let doubleNumbers = numbers.map(doubleNumber); //2 4 6 8 10 12

reduce()reduces a list of values to one value.

function addNumber(total, value){ return total + value; } function sum(...args){ return args.reduce(addNumber, 0); } sum(1,2,3); //6

Closure is an inner function that has access to the parent function’s variables, even after the parent function has executed. Look at the next example:

function createCount(){ let state = 0; return function count(){ state += 1; return state; } } let count = createCount(); console.log(count()); //1 console.log(count()); //2

count() is a nested function. count() accesses the variable state from its parent. It survives the invocation of the parent function createCount().count() is a closure.

A higher order function is a function that takes another function as an input, returns a function, or does both.

filter(), map(), reduce() are higher-order functions.

A pure function is a function that returns a value based only of its input. Pure functions don’t use variables from the outer functions. Pure functions cause no mutations.

In the previous examples isEven(), doubleNumber(), addNumber() and sum()are pure functions.

Conclusion

The power of JavaScript lies in its simplicity.

Knowing the JavaScript fundamentals makes us better at understanding and using the language.

Learn functional React, in a project-based way, with Functional Architecture with React and Redux.

Discover Functional JavaScript was named one of thebedste nye funktionelle programmeringsbøger fra BookAuthority !

For mere om anvendelse af funktionelle programmeringsteknikker i React, se på Functional React.

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