Nodejs:- Variable declaration

Background

Two weeks ago or so, I ran into a need for a sample code.

I googled based on the scenario, and interestingly enough the most prevalent sample code was one written in nodeJS.

I do not know nodeJS and could not understand the code.

I thus googled for language-specific sample code.

I found one written in C.

My C is both sloppy and sluggish, yet I managed through.

 

NodeJS

It is time to learn NodeJS.

 

Variable Declaration

 

Traditional

As I looked through sample codes online,  I was surprised that folks were declaring regular variables using the const keyword.

I was taken back.

In traditional languages, we do not use the const keyword liberally.

 

Options

Found out that there are a few ways for declaring variables in NodeJS.

In the blog post I read, one can declare variables using const, let, and var keywords.

 

Keyword Sample Usage Constraint
const const machineName = os.hostname(); The const keyword can only be used once.
The variable is declared and the value is set within the same statement.
The same variable can not be redeclared in later statements.
Also, the same variable cannot be set to a value in later statements.
let Let machineName = os.hostname(); The let keyword can only be used once.
The variable is declared and its value set within the same statement.
In subsequent statements, the variable’s value can be set to a new value.
The same variable can not be redeclared in later statements.
The same variable can be set to a new value in later statements.
var Var machineName = os.hostname(); The variable keyword can be used multiple times.
Using the var keyword, the variable can be redeclared and its value set in subsequent statements.
None applicable.

 

Sample Code

const

Success

// Allocating os module
const os = require('os');

//declare and set variable machineName
//getting machine name from os class
const machineName = os.hostname();

// Printing Machine Name
console.log("My machine name is " + machineName);

 

Failure – Identifier has already been declared
Outline
  1. Declare variable machineName using const
    • const machineName = “”;
  2. Attempt to use const again
    • const machineName = os.hostbame();
Code

// Allocating os module
const os = require('os');

//declare variable machineName
const machineName = "";

//declare and set variable machineName
//getting machine name from os class
const machineName = os.hostname();

// Printing Machine Name
console.log("My machine name is " + machineName);

 

Output – Image

 

Output – Text

/home/runner/2wwffcfi1n/index.js:7

const machineName = os.hostname();
      ^
SyntaxError: Identifier 'machineName' has already been declared


Explanation
  1. If we attempt to use const on the same variable that we have already applied it on, we will get an error message
  2. The error message will read
    • SyntaxError:- Identifier has already been declared

 

Failure – Assignment to constant variable
Outline
  1. Declare variable machineName using const
    • const machineName = “”;
  2. Attempt to set variable’s value
    • machineName = os.hostbame();
Code

// Allocating os module
const os = require('os');

//declare and set variable machineName
//getting machine name from os class
const machineName = os.hostname();

/set variable's value
machineName = os.hostname();

// Printing Machine Name
console.log("My machine name is " + machineName);

 

Output – Image

 

Output – Text

TypeError: Assignment to constant variable.
    at Object.<anonymous> (/home/runner/2wwffcfi1n/index.js:8:13)
    at Module._compile (internal/modules/cjs/loader.js:999:30)


Explanation
  1. If we attempt to set the value of a variable we have previously declared using const, we will get an error message
  2. The error message will read
    • TypeError:- Assignment to constant variable

let

Success

// Allocating os module
const os = require('os');

//declare variables
let machineName = "";

//get machine name
machineName = os.hostname();

// Printing Machine Name
console.log("My machine name is " + machineName);

 

Failure – Identifier has already been declared
Outline
  1. Declare variable machineName using let
    • let machineName = “”;
  2. Attempt to use const again
    • let machineName = os.hostbame();
Code

// Allocating os module
const os = require('os');

//declare variable machineName
let machineName = "";

//declare and set variable machineName
//getting machine name from os class
let machineName = os.hostname();

// Printing Machine Name
console.log("My machine name is " + machineName);

 

Output – Image

 

Output – Text

/home/runner/sd5la3h45jp/index.js:8
let machineName = os.hostname();
^

SyntaxError: Identifier 'machineName' has already been declared

Explanation
  1. If we attempt to use let on the same variable that we have already applied it on, we will get an error message
  2. The error message will read
    • SyntaxError:- Identifier has already been declared

var

Success

// Allocating os module
const os = require('os');

//declare variables
var machineName = "";

//get machine name
machineName = os.hostname();

// Printing Machine Name
console.log("My machine name is " + machineName);

 

SourceCode Control

GitLab

Get Machine Name

Link

  1. Variable Declaration
    • const
      getMachineName.variable.declaration.using.const.js
    • let
      getMachineName.variable.declaration.using.let.js
    • var
      getMachineName.variable.declaration.using.var.js

Summary

Hopefully by understanding the various options for declaring variables, one can make better choices by following conventional programming standards and styles.

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