what is the importance of putting semicolon always as a best practice in JavaScript

I know probably what you are thinking , we do we need to specify semicolon in my code explicitly? I am getting the right output always without specifying a semicolon also. Well it’s not always right.

Did you think Javascript Engine Syntax parser doesn’t need a semicolon?
Javascript Engine Syntax Parser always need a semicolon. If you will not put a semicolon , the syntax parser will automatically do it for you. Means , wherever syntax parser expects a semicolon , it will put one for you.

Let me give an example the importance of semicolon. Consider the below code

What will be the output of this function? everyone might think output will be {name: "Jameel"}
But it’s not right , the output is undefined .

When you put object literal in the next line of return statement , syntax parser will do a automatic semicolon insertion after the keyword return and jump out of the function.

How can we Fix this issue?
We have to prevent the automatic semicolon insertion by doing like below

Now you will get the output of
{name: "Jameel"}

Is Javascript Object Literal same as JSON?

There are lot of people still confused about Javascript Object versus and JSON.

both are same?

Answer is No . There is a difference between two

Look at the below sample code , both objects are valid javascript object literal.
var employeeObject1 = {
firstName: "Jameel",
lastName: "Moideen"
}

var employeeObject2 = {
"firstName": "Jameel",
"lastName": "Moideen"
}

Then what is the difference between JSON?
Answer is very simple. JSON have very strict rule , the property name should enclose with quotes.In the above example employeeObject2 is valid JSON.

Valid JSON
{
“firstName”: “Jameel”,
“lastName”: “Moideen”
}

InValid JSON
{
firstName: “Jameel”,
lastName: “Moideen”
}

There are some built in function in javascript to convert an array in to JSON , which can use from object to Valid JSON before you send it across the wire.

JSON.stringfy() – Convert an object to valid JSON
Example
JSON.stringify(employeeObject1)

“{“firstName”:”Jameel”,”lastName”:”Moideen”}”

JSON.parse() – Convert a string to Javascript Object
JSON.parse('{"firstName":"Jameel","lastName":"Moideen"}')

{firstName: “Jameel”, lastName: “Moideen”}

JavaScript check null or undefined – Best Practice

Let’s assume I have a variable called ‘a’ , which looks for some value.
var a;
if (a !== undefined && a!== null && a!=='') {
console.log("Something");
} else {
console.log("Nothing");
}

I have seen lot of developers is trying check condition like above to ensure we are getting the expected value we want.Do we really need to check the value a undefined and null and ?.

No. We don’t need.

Do like below as Best Practice

var a;
if (a) {
console.log("Something");
} else {
console.log("Nothing");
}

Did you understand what’s Javascript Engine doing under the hood?. In order to understand, Let’s analyze what will be the value return by the Javascript Engine when converting undefined , null and ”(An empty string also). You can directly check the same on your developer console.

You can see all are converting to false , means All these three are assuming ‘lack of existence’ by javascript. So you no need to explicitly check all the three in your code.

Also I want to point out one more thing

What will be the result of Boolean(0)?

Ofcourse false. This will create some bug on your code when 0 become a valid value on your expected result. So please make sure this thing also when you write the code

How to subscribe and unsubscribe the window load event in Rxjs

I know every web programmers are aware of adding window.addeventlistener to ensure some codes are executing once when loading the web page. You can remove the listener by removing using removeEventListener to make avoid the memory leak.

You can do the same thing in a RxJs fashion using take(1). So you no need to unsubscribe it explicitly to avoid the memory leak.
var onWindowLoad=Observable.fromEvent(window,'load');

var onLoadSubscription=onWindowLoad.take(1).forEach(function(event){
alert("onLoadFireEvent");
});

How Map and Filter function works in RXJS

In order to understand how Map and Filter works , we can implement our own map function.
Map
Map in Rxjs used for projection, means you can transform the array in to entirely new array.

Array.prototype.map = function(projectionFunction){
var results=[];
this.forEach(function(item) {
results.push(projectionFunction(item));
});
return results;
};

You can see I have written a map function which accepts an anonymous function as a parameter. This will be your function to apply the projection to transform the array. Inside the map function you can see iterating each item in a array , call the project function by passing each item and finally the result of the projection function will push to the results array.

JSON.stringify([1,2,3].map(function(x){return x+1;}))

Output
[2,3,4]

https://www.learnrxjs.io/operators/transformation/map.html

Filter
Filter is used to filter an array based on some condition. You can specify the condition through an anonymous function. The implementation of Filter and Map are almost same only the difference is Map will transform each item in an array and will push to the main array. But the filter will only push to the main array if the condition pass.
var hotelCollection = [
{
"id": 9999,
"name": "Hotel XXX",
"place": "Wayne",
"rating": 5.0
},
{
"id": 8888,
"name": "Hotel YYY",
"place": "WoodCliff Lake",
"rating": 4.5
},
{
"id": 7777,
"name": "Hotel ZZZ",
"place": "Franklin Lake",
"rating": 5.0
},
{
"id": 6666,
"title": "Hotel PPP",
"place": "Lancester",
"rating": 4.3
},
];

Source

Array.prototype.filter=function(predicateFunction){
var results=[];
this.forEach(element => {
if(predicateFunction(element)){
results.push(element)
}
});
return results;
}

var collection = hotelCollection.filter(x=>x.rating===5);
https://www.learnrxjs.io/operators/filtering/filter.html
Thanks for reading

defineProperty method in Javascript

defineProperty is a method on Object which allow you to configure the properties to meet some criterias.
Here is a simple example with an employee object with two properties firstName & lastName and append the two properties by overriding the toString method on the object.
You will get Output as : Jameel Moideen

I am going to change the same code by using defineProperty on the object

The first parameter is the name of the object and then second parameter is name of the property we are adding , in our case it’s toString and then the last parameter is json object which have a value going to be a function and three parameters writable,enumerable and configurable.Right now I just declared everything as true.

If u run the example you will get Output as : Jameel Moideen

Let’s understand why we need the three properties such as writable,enumerable and configurable.
writable
One of the very annoying part of the javascript is , if you change the toString property to something else for example

if run this again , everything gets breaks
Let’s change writable to false. If run the same again you will get the correct output as ‘Jameel Moideen’ . This property will prevent overwrite this property later.
enumerable
if you print all the keys inside the object , you can see all the properties including toString.
console.log(Object.keys(employee));
if you set enumerable to false , you can hide toString property from everybody else. If run this again you will get firstName,lastName
configurable

if someone later redefined the object on later for example enumerable to true and run it. You can see toString property came again.


we no need this behavior , so you can restrict this by set configurable to false.