Variable scope is defining how you can and can't access places in your code. The two types are global and block. Global code is usually avoided since it can be available anywhere in your code. Block code is basically coding with a block that limits a variables access. When using javascript if you go into strict mode any variable made without "let" or "const" will be met with an error message. Side effects come from a function reaching out and becoming "global" in a way. A pure function is when you code with no side effects. Side effects aren't always bad but can be a menace.