Confused by var, let, and const? Let's Clear the Mess

Ah, JavaScript variables! If you’ve ever found yourself scratching your head over the differences between var, let, and const, you’re not alone. Understanding these three can feel like trying to navigate a maze. But fear not, dear coder! I am here to clarify the confusion.

Basic Concept : Assigning and Redeclaration of Variable

Let's start with 'Var' - The Old Guard of JS

The variable associated with 'var' keyword can be reassign and redeclared.

'Let' - The Modern contender

The variable associated with 'let' can be reassign but cannot be redeclared.

'Const' - The Immutable Force

The variable associated with 'const' can neither be reassign not redeclared.

If you are a beginner and want to know about the bare minimum about variables, then this STOP is for you. Below we will discuss some advance concepts related to variables which will come in handy like Scoping and Hoisting .

Scoping - Accessibility of variables within specific parts of code.

Where 'var' has Function Scope but 'let' and 'const' are Block Scope. It means 'var' can be accessed throughout the function even in blocks (if,else etc..) And 'let' and 'const' can be accessed inside its own Block.

Hoisting - In simple words, it means using variables without declaring them is called Hoisting.

'var' declaration is hoisted and initialized as 'undefined', you can use the variable before its declartion but its value will be undefined.
'let' and 'const' declaration is hoisted but not initialized, you cannot use the variable before its declaration, attempting to do so will result in 'reference error'.

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Comments

[Florian Rappl]

Immutable force? You even continue to describe it correctly. Maybe call it something else - but const is not immutable. It just cannot be re-assigned (that implies that re-declarations are not possible, too).

Mutable:

const obj = { a: 9 };
obj.a = 5;
console.log(obj.a); // 5

Compare that with, e.g., Object.freeze, which truly makes it immutable (ignores any modification attempts):

const obj = Object.freeze({ a: 9 });
obj.a = 5;
console.log(obj.a); // 9

[Sultan Akhter]

If you are referring to the case of 'const' in Object and Arrays, in which the values can be modified. Then you are correct, const isn't an immutable force.

[Kat]

Was literally reading into this last night for my code. Thank you for the write up, it helped clear a few things up for me :)

[Sultan Akhter]

I am Glad, it helps.