Mastering JavaScript Functional Programming - Second Edition

Referential transparency

In mathematics, referential transparency is the property that lets you replace an expression with its value without changing the results of whatever you were doing.

The counterpart of referential transparency is, appropriately enough, referential opacity. A referentially opaque function cannot guarantee that it will always produce the same result, even when called with the same arguments.

To give a simple example, let's consider what happens with an optimizing compiler that performs constant folding. Suppose you have a sentence like this:

const x = 1 + 2 * 3;

The compiler might optimize the code to the following by noting that 2 * 3 is a constant value:

const x = 1 + 6;

Even better, a new round of optimization could ...