Understanding the Const Keyword in ES6

In ECMAScript 6 (ES6), the keyword that allows for declaring read-only named constants is const. Unlike var and let which declare variables, const is used to declare a constant - a value that, once declared, cannot be changed.

Practical Examples

To understand how const works, let's look at some examples. Here's how you would typically use const:

const PI = 3.14159;

In this example, PI is a constant that holds the value of pi to 5 decimal places. Once declared, the value of PI cannot be changed. So, if you try to reassign a value to PI:

PI = 3.14; // This will throw an error

The JavaScript engine will throw an error because you’re trying to change a read-only constant.

Difference Between let, var and const

Understanding the difference between let, var, and const is vital in JavaScript programming.

• var: Prior to ES6, developers could only use var to declare variables. var is function-scoped, which means if a variable is declared within a function using var, it can't be accessed outside that function.

• let: Introduced in ES6, let allows developers to declare block-scoped variables. They can be updated but not re-declared.

• const: Also introduced in ES6, const is similar to let but is read-only. Once the value is set, it cannot be changed.

While var, let, and const all have their uses, best practices suggest using let and const as much as possible and limiting the use of var. Because let and const are block-scoped, they help to prevent bugs that arise from the function-scoped nature of var.

In Conclusion

The const keyword in ES6 allows programmers to declare read-only constants, providing a way to define a value that shouldn’t be changed. However, remember to use these ES6 features wisely and according to the best practices in your JavaScript programs.