Javascript observer

That implements the Subject interface: // Subjectclass Store implements Subject {} Next, initialize the Subject’s state in the Store class. The subject’s observers will react to changes to this state. In this case, the state is a number, and the observers will react to an increase in the number: // Subject stateprivate numberOfProducts: number; Next, initialize an array of observers. This array is how you'll keep track of the observers: // initializing observersprivate observers: Observer[] = []; You might find some implementations of the observer pattern using a Set data structure in place of an array to keep track of the observer. Using a Set will ensure that the same observer won’t appear twice. If you want to use an array instead, you should check for duplicate observers in your attach method. Next, you should implement the Subject’s methods—attach, detach, and notify/update—in your concrete class. To implement the attach method, first check if the observer is already attached and throw an error if it is. Otherwise, add the observer to the array using the JavaScript array method, push: // Attaching Observer(s)attachObserver(observer: Observer): void { // Check if the observer has already been attached const observerExists = this.observers.includes(observer); if (observerExists) { throw new Error('Observer has already been subscribed '); } // Add a new observer this.observers.push(observer);} Next, implement your detach method by finding the index and removing it from the array using the JavaScript splice method. There can be scenarios where the observer you are trying to detach has already been detached or was not subscribed in the first place. You should handle these scenarios by adding a conditional statement to check if the observer is in the array or the set as the case may be. // Detaching Observer(s)detachObserver(observer: Observer): void { console.log(`Detaching observer ${JSON.stringify(observer)}`); const observerIndex = this.observers.indexOf(observer); if

A design pattern is a template that solves a commonly recurring problem in software design. The observer pattern, also known as the publish-subscribe pattern, is a behavioral pattern. It allows you to notify multiple objects or subscribers about any event that is published in the object they’re observing. Here you will learn how to implement the observer design pattern in TypeScript. The Observer Pattern The observer pattern works by defining a one-to-many relationship between the publisher and its subscribers. When an event occurs in the publisher, it will notify all subscribers to that event. One widespread example of this pattern is JavaScript event listeners. For context, assume you’re building an inventory tracker that keeps track of the number of products in your store. In this case, your store is the subject/publisher, and your inventory is the observer/subscriber. Using the observer design pattern would be optimal in this situation. In the observer design pattern, your subject class must implement three methods: An attach method. This method adds an observer to the subject. A detach method. This method removes an observer from a subject. A notify/update method. This method notifies the subject’s observers when the state changes in the subject. Your observer class must implement one method, the update method. This method reacts when there’s a change in its subject’s state. Implementing the Subject and Observer Classes The first step to implementing this pattern is to create interfaces for the subject and observer class, to ensure that they implement the correct methods: // Subject/Publisher Interfaceinterface Subject { attachObserver(observer: Observer): void; detachObserver(observer: Observer): void; notifyObserver(): void;} // Observer/Subscriber Interfaceinterface Observer { update(subject: Subject): void;} The interfaces in the code block above define the methods your concrete classes must implement. A Concrete Subject Class The next step is to implement a concrete subject class

Like plain JS objects.import { observable } from '@nx-js/observer-util';const counter = observable({ num: 0 });// observables behave like plain JS objectscounter.num = 12;ReactionsReactions are functions, which use observables. They can be created with the observe function and they are automatically executed whenever the observables - used by them - change.Vanilla JavaScript console.log(counter.num));// this calls countLogger and logs 1counter.num++;">import { observable, observe } from '@nx-js/observer-util';const counter = observable({ num: 0 });const countLogger = observe(() => console.log(counter.num));// this calls countLogger and logs 1counter.num++;React Component {counter.num});">import { store, view } from 'react-easy-state';// this is an observable storeconst counter = store({ num: 0, up() { this.num++; }});// this is a reactive component, which re-renders whenever counter.num changesconst UserComp = view(() => div onClick={counter.up}>{counter.num}div>);Preact Component{store.title} }}">import { observer } from "preact-nx-observer";let store = observable({ title: "This is foo's data"});@observer // Component will now re-render whenever store.title changes.class Foo extends Component { render() { return h1>{store.title}h1> }}More examplesDynamic properties console.log(profile.name));// logs 'Bob'profile.name = 'Bob';">import { observable, observe } from '@nx-js/observer-util';const profile = observable();observe(() => console.log(profile.name));// logs 'Bob'profile.name = 'Bob';Nested properties console.log(`${person.name.first} ${person.name.last}`));// logs 'Bob Smith'person.name.first = 'Bob';">import { observable, observe } from '@nx-js/observer-util';const person = observable({ name: { first: 'John', last: 'Smith' }, age: 22});observe(() => console.log(`${person.name.first} ${person.name.last}`));// logs 'Bob Smith'person.name.first = 'Bob';Getter properties console.log(person.name));// logs 'Ann Smith'person.firstName = 'Ann';">import { observable, observe } from '@nx-js/observer-util';const person = observable({ firstName: 'Bob', lastName: 'Smith', get name() { return `${this.firstName} ${this.lastName}`; }});observe(() => console.log(person.name));// logs 'Ann Smith'person.firstName = 'Ann';Conditionals { if (person.gender === 'male') { console.log(`Mr. ${person.name}`); } else { console.log(`Ms. ${person.name}`); }});// logs 'Ms. Potato'person.gender = 'female';">import { observable, observe } from '@nx-js/observer-util';const person = observable({ gender: 'male', name: 'Potato'});observe(() => { if (person.gender === 'male') { console.log(`Mr. ${person.name}`); } else { console.log(`Ms. ${person.name}`); }});// logs 'Ms. Potato'person.gender = 'female';Arrays console.log(users.join(', ')));// logs 'Bob'users.push('Bob');// logs 'Bob, John'users.push('John');//

Reading Time: 5 minutesSometimes you may run into a challenging situation: You want some JavaScript to execute, but only if some other element has changed. At work, we’ve recently run into a situation with this kind of challenge. We’re using a library on videos, but how can we make sure that our JavaScript only runs after the video library runs? Well, we do it by mixing two really cool technologies: MutationObserver and Promise.Learning the MutationObserverSo what is a MutationObserver ?It’s an interface that allows you to observe a mutation in the DOM1. It’s a way to find out if elements have changed. We can discover if the element has gotten new attributes, or if it’s had child elements added or removed. So that makes this super handy for those situations when things out of our control are messing with our DOM.First, make an observerlet’s suppose you have a video, and you want to know if that video gets any new attributes added to it. First, we make a new observer.const observer = new MutationObserver((mutations) => { mutations.forEach((mutation) => { console.log(mutation); // do something! });});A few things to note here:The argument passed in to your new MutationObserver is a callbackYou will need to iterate over the mutations to find the mutations you wantSecond, make a configurationBefore we start observing anything, we need to give some instructions on what to observe. You have a pretty good selection on how to configure your observer. With the exception of attributeFilter, all of these are boolean values, useful for answering questions you may have about the element:childListHave elements been added/removed directly in this element?attributesHas the element had any changes to attributes?characterDataHas any text inside of the element changed?subtreeHave elements more than one level deep changed?attributeOldValueDo you want the original value of the attribute?characterDataOldValueDo you want the original text of the element?attributeFilterWhat specific attributes should be watched?All we want to know is if a class name is changing. Easy enough!const observerConfig = { attributes: true, attributeOldValue: true};Next step: start observingWe’ve created the observer. We’ve created a configuration for it. Now, we just want to put it to useobserver.observe( document.querySelector('.myVideo'), observerConfig);We use the observe method, and pass in two arguments:Element to observeConfiguration to useBut it feels kinda weirdIf you’re thinking, “neat, but it feels weird,” you’re not alone. My long experience with JavaScript in the DOM tells me I should be able to do something like this:document .querySelector('.video') .addEventListener('mutation',mutationCallback)But, how would we be able to decide which mutations to watch (i.e. pass in a configuration)?¯_(ツ)_/¯Maybe that’s why it’s not an event to listen to on an element.And then the promiseSimply put, a promise is an object that may produce a single value at some time in the future2. I like to think of promises as an eventuality. Eventualities are events that will happen, but you don’t have the details of what happens. So a promise is a way to make sure that something always happens. This makes promises exceptionally helpful for AJAX. But it also makes them

Different user types (cycling, driving, or running).This code block demonstrates the strategy design pattern in JavaScript code://situation: Build a calculator app that executes an operation between 2 numbers.//depending on the user input, change between division and modulus operationsclass CalculationStrategy { performExecution(a, b) {}}//create an algorithm for divisionclass DivisionStrategy extends CalculationStrategy { performExecution(a, b) { return a / b; }}//create another algorithm for performing modulusclass ModuloStrategy extends CalculationStrategy { performExecution(a, b) { return a % b; }}//this class will help the program switch between our algorithms:class StrategyManager { setStrategy(strategy) { this.strategy = strategy; } executeStrategy(a, b) { return this.strategy.performExecution(a, b); }}const moduloOperation = new ModuloStrategy();const divisionOp = new DivisionStrategy();const strategyManager = new StrategyManager();//use the division algorithm to divide two numbers:strategyManager.setStrategy(divisionOp);var result = strategyManager.executeStrategy(20, 4);console.log("Result is: ", result);//switch to the modulus strategy to perform modulus:strategyManager.setStrategy(moduloOperation);result = strategyManager.executeStrategy(20, 4);console.log("Result of modulo is ", result);Here’s what we did in the above block:First, we created a base CalculationStrategy abstract class, which will process two numbers: a and bWe then defined two child classes, DivisionStrategy and ModuloStrategy. These two classes consist of division and modulo algorithms and return the outputNext, we declared a StrategyManager class, which will let the program alternate between different algorithmsIn the end, we used our DivisionStrategy and ModuloStrategy algorithms to process two numbers and return their output. To switch between these strategies, the strategyManager instance was usedWhen we execute this program, the expected output is strategyManager first using DivisionStrategy to divide two numbers and then switching to ModuloStrategy to return the modulo of those inputs:ObserverThe observer design pattern allows you to define a subscription mechanism to inform subscribed objects about any events that happen to the object they’re observing. A notable example of this pattern is React’s useEffect Hook, which runs every time a certain variable mutates.Here’s a block of React code that showcases the observer pattern in action:import { useEffect, useState } from "react";function App() { const [count, setCount] = useState(0); useEffect(() => { console.log("the value of count has changed"); }, [count]); //the count variable is a dependency of the useEffect Hook // this means return ( setCount((count) => count + 1)}> count is {count} {count % 2 === 0 ? is even : Not even } );}export default App;Here’s what we’re doing in the snippet above:Initially, we declared a count state variable. We will subscribe this variable to demonstrate the observer patternLater, we used the useEffect function to log out the value of count whenever it changesWhen this code is run, the program should log out whenever count changes and whether the value is even or odd:ConclusionThis article highlighted the importance of JavaScript design patterns for creating scalable and maintainable software. We explored patterns like singleton, builder, and decorator, and