JavaScript variables can be declared using the var, let, or const keywords depending on your scoping and mutability requirements.
JavaScript variable declaration syntax
Declare variables in JavaScript using the keywords var, let, or const.
Use the equal (=) operator to assign values to variables in JavaScript.
Basic syntax examples:
var x = 5;
let y = 10;
const z = 15;
The var Declaration
The var keyword represents JavaScript's original method for declaring variables, dating back to the language's earliest versions.
Variables declared with var exhibit function scope behavior, meaning they're accessible throughout the entire function where they're declared, regardless of block boundaries.
This permissive scoping often leads to unexpected behavior, particularly when variables are declared inside loops or conditional statements but remain accessible outside those blocks.
Additionally, var allows both redeclaration and reassignment within its scope—you can declare the same variable name multiple times without error, with later declarations overwriting earlier ones.
The let Declaration
Introduced with ES6, let provides a more controlled approach to variable declaration by implementing block scope rules.
Variables declared with let remain confined to their specific block, preventing the scope leakage issues common with var.
While let variables can be updated with new values, they cannot be redeclared within the same scope, reducing naming conflicts and accidental overwrites. This keyword was specifically designed to address the problematic behaviors of var, offering developers more predictable variable behavior that aligns with scoping rules found in most other programming languages.
The const Declaration
The const keyword creates variables that maintain constant values after initialization, requiring that the variable be assigned a value at the moment of declaration.
Like let, const follows block scoping rules and prevents redeclaration within the same scope. However, const goes further by prohibiting reassignment entirely — once a value is assigned, it cannot be changed.
It's important to note that for reference types like objects and arrays, const prevents reassignment of the variable itself but still allows mutation of the object's properties or array's elements