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The concept of mutability and immutability in JavaScript is essential to understand to avoid errors. Primitive types are simple atomic pieces of data in JavaScript. Reference types are not simple atomic values but are objects that are made up of multiple properties assigned to them. Reference values are objects stored in memory and references to objects instead of dedicated places in memory, unlike primitive types. Primitive values in JavaScript cannot have anything added upon to them, they can only be re-assigned, and hence all primitive values are immutable.