Learn Const Keyword in JavaScript

The const keyword in JavaScript is used to create variables that cannot be redeclared or changed after their first assignment. This keeps the variable’s value fixed.

Example

const country = "India";
console.log(country); // Output: India
country = "Canada"; // ❌ Error: Assignment to constant variable.

Key Characteristics of const

Must Be Initialized: You must assign a value when declaring a const variable.

const country; // ❌ Error: Missing initializer in const declaration.
const country = "India"; // ✅ Correct

Cannot Be Reassigned: Once a value is assigned to a const variable, it cannot be changed.

const country = "India";
country = "Canada"; // ❌ Error: Assignment to constant variable.

Cannot Be Redeclared: Once a variable is declared as const, it cannot be redeclared.

const country = "India";
const country = "Canada"; // ❌ Error: Identifier 'country' has already been declared.

Block Scope: const is limited to the block in which it is defined, meaning it is not accessible outside of that block.

{
  const country = "India";
}

console.log(country); // ❌ Error: Uncaught ReferenceError: country is not defined.

Works with Objects & Arrays: While you cannot reassign a new array to a const variable, you can modify the contents of the array, such as adding, removing, or changing elements.

const country = { name: "India" };
country.name = "Canada";
console.log(country.name);

country = { age: 30 }; // ❌ Error: Assignment to constant variable.

Changing the content of the array is possible in const.

const countryArray = ["India", "Canada", "Argentina"];
console.log(countryArray.toString());

countryArray[2] = "France";
console.log(countryArray.toString());

When to Use Const?

• When the value should never be reassigned (e.g., API URLs, configuration values).
• When using objects or arrays where only their contents might change.
• Helps prevent accidental reassignments and improves code safety.

Best Practice

Always use const unless you need to reassign a variable. If reassignment is needed, use let.