Ayush Hooda explains a couple core principles behind functional programming:
A pure function can be defined like this:
The output of a pure function depends only on(a) its input parameters and(b) its internal algorithm,which is unlike an OOP method, which can depend on other fields in the same class as the method.
A pure function has no side effects, i.e., that it does not read anything from the outside world or write anything to the outside world. – For example, It does not read from a file, web service, UI, or database, and does not write anything either.
As a result of those first two statements, if a pure function is called with an input parameter x an infinite number of times, it will always return the same result y. – For instance, any time a “string length” function is called with the string “Ayush”, the result will always be 5.
If I got to add one more thing, it’d be the idea that functions are first-class data types. In other words, a function can be an input to another function, the same as any other data type like int, string, etc. It takes some time to get used to that concept, but once you do, these types of languages become quite powerful.