Referential transparency

Referential transparency is another concept closely related to pure functions and side-effects. A function is pure when it is free from side-effects. An expression is said to be referentially transparent when it can be replaced with its corresponding value without changing the application's behavior. For example, if we are using the following in our code:

let result = isIndexPage("/");

We know that the isIndexPage function is referentially transparent because it would be safe to substitute it for its return type. In this case, we know that when we invoke the isIndexPage function with / as an argument, the function will always return true, which means that it would be safe to do the following:

let result = true;

A pure function is a referentially transparent expression. An expression that is not referentially transparent is known as referentially opaque.