JavaScript Variable: Declaration vs. Initialization

#javascript #beginners #programming #tutorial

In programming, variable declaration and initialization are two fundamental concepts that involve creating and assigning values to variables. Here's what each term means:

Variable Declaration:

Example:

   var shameel

Declaring a variable means, that we are just creating the container at the moment.
var is the keyword that is used to declare the variable.
Name of the variable is shameel. You can give any name here considering it is not against the rules of variable in JavaScript.

Variable Initialization:

Example:

   shameel = 1

Initializing a variable means, we assign a value to it right after its creation in one command/line

You can combine variable declaration and initialization in a single step as well:

var shameel = 1

Declaration and Initialization for var, let and const

`let`:

You can declare a variable with let.
You can initialize a variable with let.
You can both declare and initialize a variable with let in a single step.

Example:

   let age; // Declaration
   age = 25; // Initialization
   let name = "John"; // Declaration and initialization in one step

`var` (not recommended in modern JavaScript):

You can declare a variable with var.
You can initialize a variable with var.
You can both declare and initialize a variable with var in a single step.

Example:

   var x; // Declaration
   x = 10; // Initialization
   var y = 20; // Declaration and initialization in one step

`const`:

You can declare a variable with const, but you must initialize it at the same time. You cannot declare a const variable without an initial value.
You cannot reassign a new value to a const variable after it's been initialized. It remains constant.

Example:

   const pi = 3.14; // Declaration and initialization in one step
   // You cannot do this: const e; // Error: Missing initializer in const declaration
   // You cannot do this: pi = 3.14159; // Error: Assignment to constant variable

In summary, you can both declare and initialize variables using let and var. With const, you must declare and initialize the variable in a single step, and it cannot be reassigned after initialization.

Side note

It's important to note that not all programming languages handle variable declaration and initialization in the same way, and the rules can vary. Some languages require variables to be explicitly declared before use, while others may allow variables to be implicitly declared upon initialization. Understanding these concepts is essential for writing correct and efficient code in any programming language.

Happy coding! 🚀👨‍💻🌐

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Top comments (2)

Ko • Nov 14 '23 • Edited

Why did you decide that "var" is obsolete? With "var" you can write code like a human and not like a machine.

function test(x) {
    var x = x ?? 10;
    var y = x * 2;
};
function test(x) {
    try {
        var data = parse(x);
    } catch(e) {};
    console.log(data);
};
async function test(x) {
    var [error, user] = await loadUser();
    if (error) {
        // ....
    };
    var [error, post] = await loadPost();
};