Pure Functions

Pure functions are functions that don't have side effects and for the same input, arguments will return the same output value(s). A side effect can be anything from mutating the value of an argument passed by reference (which in JavaScript mutates the original) to mutating the value of a local variable, or doing any sort of I/O.

A pure function can be thought of as a mathematical function. It only operates using input and only affects its own output.

Here is a simple pure function, the identity function, which returns whatever is passed to it as a parameter:

const identity = i => i;

Notice how there are no side effects and no mutation of parameters or creation of new variables. This function doesn't even have a body.

Pure functions have the advantage of being simple to reason about. They're also easy to test; there is usually no need to mock any dependencies out since any and all dependencies should be passed as arguments. Pure functions...