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Mastering JavaScript Design Patterns - Second Edition by Simon Timms

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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.

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