Pure Functions

Are your functions pure?

If you don’t know what that means, then they’re probably not.

What is a pure function?

Simply put, a pure function is a function with no side effects.

Just like allergy medicine whose commercials state that “common side-effects may include…” as rapid after-thoughts muttered faster than your elementary school teacher expected you back from the bathroom, a function with side effects can keep you up at night later down the road.

What are side effects?

In this context, side effects are non-deterministic inputs; the same input can give me a different output each time.

Common side effects may include:

Accessing the filesystem (file system could change between function calls)
Making network calls (you send a request and wait for a response, which could be different every time)
Accessing global variables (global variables could be changed by other pieces of code)
Accessing the current time (the current time is always changing)
Using a random number generator (result will always be different)

Pure functions should be easy to understand and as deterministic as possible.

For example, if I have a function that reads data from a CSV file, performs operations on the data, and then returns the result, I run the risk of different results every time I call the function.

If I separate the function into: One function which reads a file and returns the data called readCSVInput(filePath) Another function that converts the data into a usable format called convertCSVTextToArray(csvText) And a third that performs operations on the data and returns the results called calculateData(data), the outcome brings a myriad of benefits.

I get consistent results every time I call the “convertCSVContentToArray” or the “calculateData” function with the same parameters
Automated tests / unit tests are easier to write and rely on - I can test each function individually
The function can be re-used more easily in another part of the code
The code likely becomes more readable - I can understand 3 small functions better than 1 large function

Ask your doctor if impure functions are right for you.

Take care