In JavaScript, a constant is a read-only value that cannot be reassigned. Constants are declared using the const keyword and are block-scoped like variables declared with let.
Here is an example:
// Declare a constant
const PI = 3.14159;
// Attempt to reassign the constant (this will throw an error)
PI = 3.14; // Uncaught TypeError: Assignment to constant variable.
// Constants must be initialized at declaration
const greeting; // Uncaught SyntaxError: Missing initializer in const declaration.
// Constants are block-scoped
if (true) {
const message = 'Hello, world!';
}
console.log(message); // Uncaught ReferenceError: message is not defined
In the above example, the PI constant is initialized with a value and cannot be reassigned later. Attempting to do so will result in a TypeError. The greeting constant however will throw a SyntaxError because it is not initialized at declaration.
Constants can also be used with objects and arrays. While the constant variable cannot be reassigned, the properties or elements within the objects or arrays can still be modified.
// Declare an object constant
const person = {
name: 'John',
age: 30
};
// Modify a property in the object
person.name = 'Jane';
console.log(person); // { name: 'Jane', age: 30 }
// Declare an array constant
const numbers = [1, 2, 3];
// Modify an element in the array
numbers[0] = 4;
console.log(numbers); // [4, 2, 3]
In the above example, the person and numbers constants are object and array types respectively. While the variables themselves are immutable, the properties and elements can still be modified.