JavaScript : Const

In JavaScript, const is a keyword introduced in ECMAScript 6 (ES6) that allows developers to declare variables that are block-scoped constants. Once a variable is declared with const and assigned a value, that value cannot be reassigned or changed. Here's how const is used:

1. Declaration and Assignment:


You use const to declare a variable and assign an initial value. Once assigned, the value cannot be changed.


const PI = 3.14;
const name = "John";
 

2. Block Scope:


Like let, variables declared with const are block-scoped. They are only accessible within the block they are defined in.

Example:

{
  const x = 10;
  console.log(x);  // Output: 10
}

// x is not accessible here
console.log(x);  // Uncaught ReferenceError: x is not defined
 

3. Reassignment:


Unlike variables declared with let, variables declared with const cannot be reassigned. Trying to reassign a const variable will result in an error.

Example:

const count = 5;
count = 10;  // Error: Assignment to constant variable

4. Initialization Required:


When using const, you must initialize the variable with a value at the time of declaration. Otherwise, you'll get an error.

Example:

const x;  // SyntaxError: Missing initializer in const declaration
x = 5;
 

5. Immutable Values:


Variables declared with const are not truly immutable, but their values are read-only. This means that if a const variable holds an object or array, you can still modify the properties or elements of that object or array. However, you cannot reassign the variable itself.

Example:

const person = {
  name: "John",
  age: 30
};

person.age = 31;  // Valid, modifies the object
person = { name: "Jane", age: 25 };  // Error: Assignment to constant variable
 

6. Benefits of const:

 

  • Use const when you want to declare a variable that will not be reassigned.
  • Provides a clear signal to other developers that the variable should not be changed.
  • Helps prevent accidental reassignments, improving code safety.

Example Usage:


Here's an example demonstrating the usage of `const`:


const gravity = 9.81;  // Declaring a constant

if (gravity > 9.8) {
  const message = "High gravity";  // Scoped to this block
  console.log(message);   // Output: High gravity
}

// message is not accessible here
console.log(message);   // Uncaught ReferenceError: message is not defined
 

When to Use const:

 

  • Use const by default for variables that you know will not change their value.
  • Use const for constants like mathematical constants ( PI , E , etc.) or configuration values.
  • Use const for variables that hold object references that you don't want to change, but remember that the properties

Notes:

  • Variables declared with const are more predictable and easier to reason about, as you can trust that their values won't change.
  • If you're unsure whether a variable will need to change, start with const and switch to let if needed.

Conclusion:


const is used to declare variables that are read-only constants. It ensures that the value of the variable does not change throughout the program, providing predictability and safety. Understanding how const works is important for writing robust and maintainable JavaScript code.