Day 9: const and let

Today’s topic is rather simple than everything we have discussed so far, and we won’t be needing much time on trying to understand it.

Even thought it’s pretty simple, it’s still required to be discussed on a separate day in our DailyJS initiative because of it’s importance. After const and let were introduced, people rarely use the var keyword now, so you can understand how important these two are.

So coming on to the main topic, const and let are the ways of declaring the variables.

You might be aware of the var keyword. Earlier (and it’s still acceptable but not recommended) people used to declare variables using var keyword.

Syntax

Syntax is pretty simple, just like you declare variables in other languages, or even JavaScript with var, you can declare variables using const and let.

Some very important things about them which we should always keep in mind is that

1. They both are block scoped,
2. let can be reassigned
3. const cannot be reassigned

const

const <variable_name> = <value>

let

let <variable_name> = <value>

A Simple Example

/**
 * const let example
 */

const name = "Madhav";
let profession = "Student";
let age = 21;

console.log ("Name: ", name);
console.log ("Profession: ", profession);
console.log ("Age: ", age);

// Let's say 1 year passed and his profession changed
// We can reassign 
//  name = "Madhav new"; -> This will give an error
profession = "Software Engineer";
age = 22;

console.log ("/* ===== After a year ===== */");console.log ("Name: ", name);
console.log ("Profession: ", profession);
console.log ("Age: ", age);

What’s the problem with var though?

To be honest, there were no major problems with var, but there were some possibilities of “messing things up”.

I know it might sound funny, but it’s true. Let’s consider this example,

var name = "Madhav"
console.log (name); // Madhav
var age;
console.log (age); // Undefined. No errors

Doing anything with uninitialized variables should give an error, but it won’t

Let’s see another example, var is not block scoped, before const and let, variables were either having global scope or function scope, not block scope.

var animal1 = "dog";
if (animal1 === "dog") {
    var animal1 = "cat";
}
console.log (animal1); // Should give "dog", but prints "cat"

Let’s solve both the issues adressed above

Issue 1 - solved using const

var name = "Madhav"
console.log (name); // Madhav
var age;
console.log (age); // Undefined. No errors

const name1 = "Madhav";
console.log (name1); // Madhav
const age1;
console.log (age1); // Error!

Issue 2 - Solved using block scoped let and const

/**
 * var is not block scoped
 * let and const are block scoped
 */

// #1: var
var animal1 = "dog";
if (animal1 === "dog") {
    var animal1 = "cat";
}
console.log (animal1); // cat "Ohh no!"

// #2: let
let animal2 = "dog";
if (animal2 === "dog") {
    let animal2 = "cat";
}
console.log (animal2); // dog "Perfect :)"

// #3 const 
const animal3 = "dog";
if (animal3 === "dog") {
    const animal3 = "cat";
}
console.log (animal3); // dog "Perfect :)";

Properties of const

Before ending today’s topic, it’s important to put forward some important properties of const so that you don’t mess things up while using it in future.

const can be seen as somewhat similar to constant in other languages.

1. const is read-only
2. It can’t be redeclared
3. It can’t be reassigned
4. Even so, it is mutable, Object and arrary values can be modified.

That’s it. See you tomorrow :)

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