DEV Community: sanjeev

javascript variable declaration: var vs let vs const

sanjeev — Fri, 18 Oct 2019 11:07:49 +0000

We have 3 different ways to declare variables in java script

var - Variables declared with var keyword are function scoped. What do I mean by that ? let's take an example -
```
function sayHello(){
  for(var i=0;i<4;i++){
    console.log(i);
  }
  // As var is function scoped, i is available outside for loop.
  console.log(i); // This prints 4.
}
sayHello();
```
Here, i is declared with var keyword in a for loop ( block ) and it is incremented util i < 4. When i becomes 4, the for loop ends and the last console.log(i) is executed. Output
```
0
1
2
3
4
```
Because of the var keyword, i is function scoped hence i is available outside the for loop as well and last console.log() prints 4.
let - This is part of ES6 and addresses the problem related to var keyword. Variables declared with let keyword are block scoped. Let's consider the previous code with let
```
function sayHello(){
  for(let i=0;i<4;i++){
    console.log(i);
  }
  // let is block scoped hence i is only visible inside for block.
  console.log(i); // This line will throw an error.
}
sayHello();
```
Because i is declared with let, i is visible inside the for loop only and the last console.log() will throw an error ReferenceError: i is not defined Output
```
0
1
2
3
Uncaught ReferenceError: i is not defined
    at sayHello (:6:15)
    at :8:1
```

const - const keyword is used to define constants in java scripts and are block scoped. e.g.


function sayHello(){
    const i = 1;
    console.log(i);// This prints 1
    i = 2;  // i is declared as constant hence this line will throw an error.
}
sayHello();

if we try to reassign i with a different value within the same block then java script engine will throw an error TypeError: Assignment to constant variable Output


1
Uncaught TypeError: Assignment to constant variable.
    at sayHello (:4:4)
    at :6:1

Conclusion: With var keyword we may get unexpected output, so if the value of a variable is not expected to change then const keyword should be preferred else use let keyword to define variables.

Cheers!

javascript question - Hoisting

sanjeev — Tue, 15 Oct 2019 11:15:48 +0000

What you think, will be the output of the following code ?

hello(); console.log(a); function hello(){ console.log("Hello World!"); } var a = 5; console.log(a);

A small hint, the output of the above code snippet depends on : -
Hoisting - An interesting javascript concepts where variable and function declarations are allocated memory during context creation phase (phase 1) before execution

Javascript engine will start executing code line by line in the execution phase (phase 2)

Cheers!

Data Structures and Algorithms Visual Algo

sanjeev — Tue, 15 Oct 2019 04:18:52 +0000

Data structures and Algorithms are one of the most important topics in computer science.

One of main challenges I personally have faced while learning Data structures and Angorithms is to visualize the steps while performing operations like insert, lookup, remove etc.

Recently I came across a wonderful website which has helped me to understand DSA by providing animated visuals

Please find below url -

https://visualgo.net/en

Please take a look and I hope you will also find this helpful.

Cheers!