Mutating a value. When you receive a data structure that is in its identity mutable, like an array or object, the best thing to do is to always assume that it is a read-only data structure and that you are not allowed to mutate it. Make a copy of the object/array (ex. spread operator) and then mutate it locally, so you don’t create a side effect on the outside program.

Preventing a value mutation. If you are calling a function and you are going to pass to it a data structure like an array or object, make it clear to the reader that you don’t want it to be changed. Use something like Object.freeze method in JavaScript. Create a helper function that you can use to “freeze” an object so no new properties can be added, changed or removed.