Learning TypeScript 2.x - Second Edition

Pure functions

FP introduces some concepts and principles that will help us to improve the predictability of our code. Let's look at one of these core concepts: pure functions.

A function can be considered pure when it returns a value computed using only the arguments passed to it. Also, a pure function avoids mutating its arguments or any other external variables. As a result, a pure function always returns the same value given the same arguments, independently of when it is invoked.

The isIndexPage function declared in the preceding section is not a pure function because it accesses the pathname variable and it has not been passed as an argument to the function. We can transform the preceding function into a pure function by rewriting it ...