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Function Declaration To declare a function follow the syntax: function function_name(parameter1, parameter2, ..., parameterN) { // Body of the function } The function can have access to outer variable, as well as modifying it. The outer variable is only...

Objects In JavaScript objects are used to store key to value pair collection of data. You can think of them as dictionary in Python. You can create an object using brackets { }, and a list of optional properties that the object will have. For example: let e...

Reachability In JavaScript garbage collection is implemented through something called reachability. Variable that are reachable are kept in memory and not deleted by the garbage collector. A value is considered to be reachable if it's reachable from a root ...

Constructor function Function that is meant to be a constructor are named with capital letter first and be executed with the new operator. We use a constructor because it creates a shorter and simpler syntax compared to creating object with {...} every time....

Optional chaining There is this problem that if the object that you are accessing might or might not contain an object attribute say address and you can trying to access address's attribute further say user.address.street, then your JavaScript will crash beca...

Symbols In JavaScript there is two primitive types that can be used as object property key String Symbol type Don't confuse the symbol type in Ruby with JavaScript they are different albeit similar in that they are used to create unique identifier. A ...

Array Two ways of creating empty array let arr = new Array(); let arr = []; Create an array with elements in them let arr = ["Apple", "Orange", "Plum"]; Array in JavaScript is heterogeneous, you can store element of any type in them. arr.length property...

Iterables A generalization of arrays, it allows us to make any object iterable in the context of for ... of loop. Making an object iterable just requires you to add the Symbol.iterator hidden property which is a function that returns the iterator. Really dou...

Map Very similar to an object, however, with object the only key that is allowed is a String. Map on the other hand allows keys of any type. Any type as it ANY type, even an object can be used as a key new Map(): Creates a new empty map map.set(key, value...

Destructuring assignment A special syntax to let us unpack either array or object into different variables Array destructuring let arr = ["Ricky", "Lu"] let [firstName, lastName] = arr; console.log(firstName); // "Ricky" console.log(lastName); // "Lu...

JSON file When you want to transport say a complicated object or an array of object to another compute using network. You cannot transport a complicated object just directly as "it is". You must serialize it which essentially turns the object into a stream of...

Rest parameters and spread syntax How do we make a function take in an arbitrary number of arguments? Simple we use the ... rest operator in the function header. function sum(...args) { let sum = 0; for (let arg of args) sum += arg; return sum; } ...

Virtual property In addition to the normal property that we have for objects, we can also set up virtual properties. They appears to the outside code as if they were the object's property but they are actually implemented as functions underneath. let user = ...

Prototype inheritance Every object have a special hidden property [[Prototype]] it is either null or a reference to another object. Whenever you read a property from object and if it is missing, JavaScript will automatically take it from the prototype. Sett...

Class basic syntax Besides using a constructor function there is a more advance object oriented programming construct called class in JavaScript. The basic syntax is as follows: class MyClass { constructor() {} method1() {} method2() {} } To ut...

Syntax try { // Code } catch (err) { // Error handling } finally { // Excuted always, regardless if there is any error. And is always last // It is also executed right before you return if you decide to return/throw from try and catch } Keep ...

Time before promise, callbacks There are entities in JavaScript that let you schedule asynchronous tasks, tasks that you initiate first, then they will finish later. Task like disk reading, fetching resources from network that will take arbitrary amount of t...

Promise.all This API will allow us to take in an iterable, an array of promises and return a new promise. That returned promise will only be resolved when all listed promises are resolved, the array of their results will become the result of this returned pr...

async keyword async keyword is placed before a function like so: async function f() { return 1; } It basically make your function always returns a promise. If you didn't return a promise explicitly in the function they will be wrapped in a resolved promi...

Two different standards In the browser JavaScript ecosystem, JavaScript modules depends on import and export statements to load and export ES modules. In addition, ES module is the official standard format to package JavaScript code for reuse. On the other ...