Understanding Constant Variables in TypeScript

In TypeScript, you can declare a variable whose value can never be changed using the const keyword. This is a feature that is also present in JavaScript ES6 from which TypeScript is derived.

The const keyword stands for "constant". A constant is a value that cannot be altered after it has been initialized. This is different from a let variable, which can be reassigned.

Below is an example of how to declare a constant in TypeScript:

const PI = 3.14159;

In the above example, we have declared a constant named PI and assigned it the value of 3.14159. If we try to assign a new value to PI, TypeScript will throw an error.

PI = 3.14;  // This will throw an error

However, using const to declare objects does not make the object immutable. You can still add properties to the object or modify its properties. The const keyword only ensures that the variable will always refer to the exact same object.

const myObject = {
  property1: "hello"
};

myObject.property1 = "world"; // This will not throw an error
myObject.property2 = "TypeScript";  // This will not throw an error

It's useful to declare a variable as a constant when you know that its value isn't going to change. This can help to prevent bugs introduced by accidental variable reassignment.

TypeScript does not support the 'final' or 'immutable' keywords for variable declaration. The const keyword is the correct way to declare a constant variable.

The readonly keyword in TypeScript is used to make the properties of a class read-only, implying they can be accessed outside the class but their value cannot be changed. While it serves a similar purpose to const keyword, it is applied in a different context and not used to declare constant variables.

While working with TypeScript, it is considered a best practice to use const whenever the value of the variable is not going to change. Use let only when the value of the variable is meant to change.