# Side effect (computer science) | | In computer science, an operation, function or expression is said to have a side effect if it modifies some state variable value(s) outside its local environment, which is to say if it has any observable effect other than its primary effect of returning a value to the invoker of the operation. Example side effects include modifying a non-local variable, modifying a static local variable, modifying a mutable argument passed by reference, performing I/O or calling other functions with side-effects. In the presence of side effects, a program's behaviour may depend on history; that is, the order of evaluation matters. Understanding and debugging a function with side effects requires knowledge about the context and its possible histories. | | ---- | ---- | | | wikipedia:: [Side effect (computer science)](https://en.wikipedia.org/wiki/Side_effect_(computer_science)) | - "A change in the state of a program made by calling a function that is not a result of reading the return value from the function. Side effects can only be produced by modifiers."