JavaScript Const

Const is a keyword that indicates a constant. It describes an entity that cannot be changed at runtime. We cannot reassign a constant. With constants we lose the ability to modify variables at runtime and we gain run-time validation.

One of the most important things is that you should try to keep things simple!

When Should You Use

When Should You Use var, const Or let?

It is important to understand Global-Scope, Function-Scope and Block-Scope to know when you may use the different statements.

Global Scope

Variables declared Globally (outside any block or function) have Global Scope.

Be careful with using Global Scope.

var car = "Byton";
// code CAN use variable car
function carFunction() {
     // code CAN also use variable car 

Function Scope

Variables declared inside a function have Function Scope.

// code can NOT use variable car
function carFunction() {
     var car = "Byton";
     // code CAN use variable car
// code can NOT use variable car

Block Scope

Variables declared inside a block {} have Block Scope.

     var car = "Byton"; 
// code CAN use variable car

Learn more about JavaScript Scope Learn more about JavaScript Hoisting

JavaScript Const

ECMAScript 2015 introduced two important JavaScript keywords: let and const.

Constants are Block-Scoped, much like variables defined using the let statement.

To keep things simple - Use const to identify a constant. Use let to define a variable.

The scope of a constant can be either global or local to the block where it is declared.

Take a look at the example to understand this.

JS JavaScript - JavaScript Const

var car = "Tesla";
const fuel = "Electric";
// Here car have the value "Tesla"
    const car = "Chrysler";
    // Here car have the value "Chrysler"
    var fuel = "petrol";   // This will give a syntax error
// Here the variable car have the value "Tesla"

Learn more about JavaScript Var Learn more about JavaScript Let

Const Assigned When Declared

Const variables must assign a value when they are declared

JS JavaScript - Const Assigned When Declared

// This is not allowed
const car;
car = "Mercedes";
// This is correct syntax
const car = "Mercedes";

Const continue

The keyword const need some more explanation.

Const defines a constant reference - it is a bit confusing.

It is not possible to change constant primitive values, but we can change the values of constant objects

Take a look at the example to understand this.

Primitive Values

A primitive const value cannot be changed

const car = "Byton";

// This will give an error
car = "Mercedes";

// This will give an error
car = car + " and Mercedes";


It is possible to change the properties of a constant object

// Create a const object
const car = {type:"Byton", model:"kbyte", color:"red"};
// It is possible to change a property
car.color = "blue";
// It is possible to add a property
car.engine = "Electric";

It is NOT possible to reassign a constant object

const car = {type:"Byton", model:"kbyte", color:"red"};
// This gives a syntax error
car = {type:"Tesla", model:"Model X", color:"red"};


It is possible to change elements in a constant array

// A constant array
const cars = ["Byton", "Tesla", "Audi"];
// It is possible to change an element
cars[2] = "Mercedes";
// It is possible to add an element

It is NOT possible to reassign a constant array

// A constant array
const cars = ["Byton", "Tesla", "Audi"];
// This gives a syntax error
cars = ["BMW", "Polestar", "Lexus"];

Learn more about JavaScript Variables Learn more about JavaScript Comments

Check this out

JS JavaScript: Comparisons

Comparison and Logical operators are used to test for true or false.

JavaScript provides three different value-comparison operations, loose equality, strict equality and They have different usage so learn when to use them.

JS JavaScript: Hoisting

JavaScript hoisting is basically that declarations are moved to the top of your code.

Actually your function and variable declarations are added to memory during the compile phase.
And they stay exactly where you typed them in your code.

JS JavaScript: Errors

There will be errors in the code. It is good practice to handle exceptions/errors.

Exceptions are a type of error that occurs during the execution of an application. Errors should be logged. Sometimes it is possible to recover a bug and proceed.
Other cases the user should get inform....

JS JavaScript: Strict Mode

The strict context prevents some actions and throws more exceptions. Strict mode changes previously accepted "bad syntax" into errors. Strict mode will for example prevent using undeclared variables. The "use strict" directive was new in ECMAScript version 5.


JavaScript is one of the 3 core language technologies of the World Wide Web that all web developers must learn:

1.      HTML to define the content of web pages.

2.      CSS to specify the layout of web pages.

3.      JavaScript to program the behavior of web pages.

JavaScript is a scripting or programming language that allows you to implement complex things on web pages. Every time a web page does more than just sit there and display static information for you to look at, displaying timely content updates, interactive maps, animated 2D/3D graphics, scrolling video jukeboxes, etc. − you can bet that JavaScript is probably involved.

The benefits of using JavaScript are:

1.      You can validate user input before sending the page off to the server. This saves server traffic, which means less load on your server.

2.      Immediate feedback to the visitors − They don't have to wait for a page reload to see if they have forgotten to enter something.

3.      Increased interactivity − You can create interfaces that react when the user hovers over them with a mouse or activates them via the keyboard.

4.      Richer interfaces − You can use JavaScript to include such items as drag-and-drop components and sliders to give a Rich Interface to your site visitors.


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