Side effects

You have probably noticed (subconsciously maybe) that print_r is a function too. It receives data of any type and prints it to screen.

Now, here is a question for you: what does print_r return? Answer: whatever it returns is not used in any way.

print_r prints something to screen, but it is just some action that a function does, rather than return of some value.

Screen output and value return from a function are different and independent operations. Technically, screen output amounts to writing a file (a special one, but nevertheless a file). This subject is be better understood when one knows how an operating system works, which is very important for a programmer.

For a program, screen output is a so-called side effect. A side effect is an action that change the outer environment (runtime environment). Such actions include any network interactions, interaction with the file system (reading and writing files), screen output of data, printing out from a printer etc. Understanding the basic idea of side effects affects the programming style and ability to build high quality programs. We teach the practical application of those principles on Hexlet.

Side effects are one of the main sources of problems and errors in program systems. A code with side effects is difficult to test and not reliable. Programming without side effects, however, doesn’t make any sense. Without them it would be impossible to get any tangible result from a program (creating a database entry, printing to screen, send some data via a network etc.).


This exercise is not directly related to the subject of the lesson, but doing it without creating variables is a very important step in your professional development.

Print to screen the name of Daenerys Targaryen’s mother, using the getParentFor function and without creating variables.

As you remember, getParentFor receives a child’s name as the first parameter and returns a parent’s name. As the second parameter it receives either father or mother. This way it knows which parent to return. By default, the second argument is set to ‘mother’.



  • Side Effect — and action that changes the outer environment (runtime environment). For example, screen output of sending an email.

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