Understanding the "const" Declaration in ES6

The introduction of ES6 (ECMAScript 2015) brought compelling new features to JavaScript, among which is the "const" declaration. Despite the common misconception, const is not exactly similar to let and neither is it specifically used for math-related variables. The correct understanding of const according to the quiz question above is that it is used to declare variables that are to remain constant throughout the program. In other words, once a variable is defined with const, you can't change its value.

The Role of "const"

The const declaration creates a read-only reference to a value. This does not mean the value it holds is immutable—just that the variable identifier cannot be reassigned. For instance:

const PI = 3.14;
console.log(PI) //Prints 3.14
PI = 99;  //TypeError: Assignment to constant variable.

In the example above, the variable PI has been declared as a constant and it’s assigned the value 3.14. When you try to reassign a new value to PI, JavaScript throws a TypeError notifying you of an assignment to a constant variable. This is because const creates an immutable variable binding.

Practical Applications

Using const makes your JavaScript code more predictable, thus reducing chances of bugs and making it easier to read and understand. It's particularly useful when you have a reference that you don’t want to change by accident like a configuration object or a function that’s supposed to stay the same.

Best Practices

While using const, bear in mind that only the assignment is immutable, not the value. So if you assign an object or an array to a const variable, the contents can still be modified:

const obj = {a: 1};
obj.a = 2;   // This will work. obj.a is now 2
console.log(obj); // {a: 2}

const arr = [1, 2, 3];
arr.push(4); // This will work. arr is now [1,2,3,4]
console.log(arr); // [1, 2, 3, 4]

In JavaScript, const doesn't mean constant state, but constant reference. This is a common area of confusion, and an important concept to understand.

Remember, const isn't a magic bullet to make everything unchangeable. It merely prevents re-assignment of a variable identifier. In the course of writing JavaScript code, ensure to use const whenever you have identifiers that shouldn’t be reassigned. This would make your code more robust and less prone to bugs.