Pure Functions for Dummies

August 26, 2019

What is a pure function

Functions that produce no side effects and have the same output given the same input are called “pure”.

You can easily check if a function is pure:

  • Pure functions always return the same output given the same input
  • Pure functions have no side-effects

Some examples:

Pure Function:

function triple(value) {
  return value * 3;
}

Impure Function:

function getCurrentTime() {
  return new Date().toLocaleTimeString();
}

The advantages of pure functions

  • Easier to test, because they are deterministic (Same input = Same output)
  • Portable, pure functions are not dependent on their environment and therefore can be more easily reused
  • Makes functions memoizable
  • Easier to refactor, because you don’t have to worry about side effects