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Is your JavaScript function actually pure

What does “pure function” mean in the context of JavaScript? In programming in general, purity is also known as “referential transparency”, a fancy way of saying “replacing an expression or function call with its result will never change the behavior of the program” or a way of saying “every time you pass the same inputs, you always get the same outputs”.

That sounds intuitive, and a function like x => x * 10 looks pure because every single time you pass it the number 3 as argument you will get 30 as output. So how can we tell that one function is pure and the other isn’t? Is it enough that we just read the code?

Spoiler: reading the code isn’t enough.


Freek Van der Herten is a partner and developer at Spatie, an Antwerp based company that specializes in creating web apps with Laravel. After hours he writes about modern PHP and Laravel on this blog. When not coding he’s probably rehearsing with his kraut rock band. He loves waffles and butterflies.