JavaScript: Functional Programming for JavaScript Developers

Immutability

One of the cornerstones of functional programming is that so called variables can be assigned only once. This is known as immutability. ECMAScript 2015 supports a new keyword, const. The const keyword can be used in the same way as var except that variables assigned with const will be immutable. For instance, the following code shows a variable and a constant that are both manipulated in the same way:

let numberOfQueens = 1;
const numberOfKings = 1;
numberOfQueens++;
numberOfKings++;
console.log(numberOfQueens);
console.log(numberOfKings);

The output of running this is the following:

2
1

As you can see, the results for the constant and variable are different.

If you're using an older browser without support, then const won't be available ...