7 JavaScript Concepts That Every Web Developer Should Know - GeeksforGeeks

13 Dec, 2021
JavaScript is Everywhere. Millions of webpages are built on JavaScript and it’s not going anywhere at least for now. On one side HTML and CSS give styling to the web pages but on the other side, it’s the magic of JavaScript that makes your web page alive. Today this language is not just limited to your web browser. You can also use it for server-side applications. Isn’t it cool to use a single language for both client-side and server-side applications? A single language fulfills both of the purposes and this is the main reason TON of job posting is there for javascript developer in the tech industry.  
According to Stack Overflow Developer Survey 2019 Javascript is #1 programming language. The language is widely used by 95% of all the websites which you can check here. Whether it’s a small startup or a big company, most of them are working on some kind of website or an app that requires a good knowledge of this language. A lot of frameworks and libraries are there for javascript. These frameworks and libraries can be easily learned if your javascript fundamentals are clear. A lot of concepts are confusing and overwhelming for developers but a good knowledge of these concepts will help you in the along run. Frameworks and libraries come and go but the fundamentals always remain the same. It’s easy to build any kind of application and learn any framework and libraries if the fundamentals are clear. Also, it will help you in interviews as well. Let’s discuss some of the basic concepts of javascript which are important to learn for any javascript developer. 
Scope means variable access. What variable do I have access to when a code is running? In javascript by default, you’re always in root scope i.e. the window scope. The scope is simply a box with a boundary for variables, functions, and objects. These boundaries put restrictions on variables and determine whether you have access to the variable or not. It limits the visibility or availability of a variable to the other parts of the code. You must have a clear understanding of this concept because it helps you to separates logic in your code and also improves the readability. A scope can be defined in two ways...  
Local Scope allow access to everything within the boundaries (inside the box)
Global Scope is everything outside the boundaries (outside the box). A global scope can not access a variable defined in local scope because it is enclosed from the outer world, except if you return it.
Example: The code given below will give you an error because “name” is defined within the boundary (local scope) of showName() function. You can not have access to this variable outside the function. 
NOTE: THE CODE BELOW IS IN ERROR DUE TO A TYPO IN THE FUNCTION CALL, causing an error before the intended scoping error is raised by the console.log call.
 
Now, take a look at the code given below and see how you can access the variable “name” defined in the local scope. 
 
 
As the name suggests IIFE is a function in javascript which immediately invoked and executed as soon as it is defined. Variables declared within the IIFE cannot be accessed by the outside world and this way you can avoid global scope from getting polluted. So the primary reason to use IIFE is to immediately executes the code and obtain data privacy.  
A lot of developers get unexpected results when they are not clear with the concept of Hoisting in javascript. In javascript, you can call a function before it is defined and you won’t get an error ‘Uncaught ReferenceError’. The reason behind this is hoisting where the javascript interpreter always moves the variables and function declaration to the top of the current scope (function scope or global scope) before the code execution. Let’s understand this with example. 
Example: Take a look at the code given below. 
 
Now what happens if we invoke our function before we declare it (with hoisting) 
 
The above code is not giving an error and you get the output ‘moo’ in your console. This is hoisting in javascript. 
Example 2: 
var a = 5;
console.log(5);
output: // 3
The above code with hoisting will give you the same output. 
a = 5;
console.log(5);
var a;
output // 5
A closure is simply a function inside another function that has access to the outer function variable. Now, this definition sound pretty much straightforward but the real magic is created with the scope. The inner function (closure) can access the variable defined in its scope (variables defined between its curly brackets), in the scope of its parent function, and the global variables. Now here you need to remember that the outer function can not have access to the inner function variable (we have already discussed this in scope concept). Let’s take an example and understand it in a better way. 
Example: 
 
In the above example, the inner function ‘second()’ is a Closure. This inner function will have access to the variable ‘greet’ which is the part of the outer function ‘first()’ scope. Here the parent scope won’t have the access of child scope variable ‘name’. 
Now the question is why do we need to learn closures? What’s the use of it? Closures are used when you want to extend behavior such as pass variables, methods, or arrays from an outer function to an inner function. In the above example, second() extends the behavior of the function first() and also has access to the variable ‘greet’. Javascript is not pure object-oriented language but you can achieve object-oriented behavior through closures. In the above example, you can think const ‘newFunc’ as an Object having property ‘greet’ and ‘second()’ a method as in an OOP language. 
Here you need to notice that after first() statement is executed, variables inside the first() function will not be destroyed (even if it has ‘return’ statement) because of closures as the scope is kept alive here and child function can still access the properties of the parent function. So closures can be defined in simple terms as “a function run, the function executed. It’s never going to execute again but it’s going to remember that there are references to those variables so the child scope always has access to the parent scope.” 
 
In javascript, a callback is simply a function that is passed to another function as a parameter and is invoked or executed inside the other function. Here a function needs to wait for another function to execute or return value and this makes the chain of the functionalities (when X is completed, then Y executed, and it goes on.). This is the reason callback is generally used in the asynchronous operation of javascript to provide the synchronous capability. 
Example: 
 
In the above example notice that greeting passed as an argument (callback) to the ‘processUserName’ function. Before the ‘greeting’ function executed it waits for the event ‘processUserName’ to execute first. 
We understand the concept of callback but what will happen if your code will have callbacks within callbacks within callbacks and it goes on. Well, this recursive structure of callback is called as ‘callback hell’ and promises help to solve this kind of issue. Promises are useful in asynchronous javascript operation when we need to execute two or more back to back operations (or chaining callback), where each subsequent function starts when the previous one is completed. A promise is an object that may produce a single value some time in the future, either a resolved value or a reason that it’s not resolved (rejected). According to developer.mozilla “A Promise is an object representing the eventual completion or failure of an asynchronous operation. Essentially, a promise is a returned object to which you attach callbacks, instead of passing callbacks into a function.”. Promises resolve the issue of ‘callback hell’ which is nothing but a recursive structure of callbacks (callbacks within callbacks within callbacks and so forth). A promise may be in three possible states... 
 
Fulfilled: When the operation is completed successfully.
Rejected: When the operation is failed.
Pending: initial state, neither fulfilled nor rejected.
Let’s discuss how to create a promise in javascript with an example. 
Example:  
Consider the above code for a sample promise assume like doing ‘isNameExist’ operation asynchronously, In that promise Object arguments as two function resolve and reject. If the operation is successful which means ‘isNameExist’ is ‘true’ then it will be resolved and display the output “User name exist” else the operation will be failed or rejected and it will display the result ‘error !’. You can easily perform chaining operations in promises where the first operation will be executed and the result of the first operation will be passed to the second operation and this will be continued further. 7. Async & Await
Stop and wait until something is resolved. Async & await just syntactic sugar on top of Promises and like promises it also provides a way to maintain asynchronous operation more synchronously. So in javascript asynchronous operation can be handled in various versions...  
ES5 -> Callback
ES6 -> Promise
ES7 -> async & await
You can use Async/await to perform the Rest API request where you want the data to fully load before pushing it to the view. For Nodejs and browser programmers async/await is a great syntactic improvement. It helps the developer to implement functional programming in javascript and it also increases the code readability. 
Example: 
 const showPosts = async () => {
 const response = await fetch('https://jsonplaceholder.typicode.com/posts');
 const posts = await response.json();
 console.log(posts);
}

showPosts();
Below is the image of when you will run this code in your console. 
To notify JS that we are working with promises we need to wrap ‘await’ inside an ‘async’ function. In the above example, we (a)wait for two things: response and posts. Before we can convert the response to JSON format, we need to make sure we have the response fetched, otherwise we can end up converting a response that is not there yet, which will most likely prompt an error.
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