Symbol is a built-in object whose constructor returns a symbol primitive — also called a Symbol value or just a Symbol — that's guaranteed to be unique. Symbols are often used to add unique property keys to an object that won't collide with keys any other code might add to the object, and which are hidden from any mechanisms other code will typically use to access the object. That enables a form of weak encapsulation, or a weak form of information hiding.| developer.mozilla.org
The spread (...) syntax allows an iterable, such as an array or string, to be expanded in places where zero or more arguments (for function calls) or elements (for array literals) are expected. In an object literal, the spread syntax enumerates the properties of an object and adds the key-value pairs to the object being created.| developer.mozilla.org
The Object.assign() static method copies all enumerable own properties from one or more source objects to a target object. It returns the modified target object.| developer.mozilla.org
The slice() method of Array instances returns a shallow copy of a portion of an array into a new array object selected from start to end (end not included) where start and end represent the index of items in that array. The original array will not be modified.| developer.mozilla.org
The Array.from() static method creates a new, shallow-copied Array instance from an iterable or array-like object.| developer.mozilla.org
The concat() method of Array instances is used to merge two or more arrays. This method does not change the existing arrays, but instead returns a new array.| developer.mozilla.org
The JSON.stringify() static method converts a JavaScript value to a JSON string, optionally replacing values if a replacer function is specified or optionally including only the specified properties if a replacer array is specified.| MDN Web Docs
A shallow copy of an object is a copy whose properties share the same references (point to the same underlying values) as those of the source object from which the copy was made. As a result, when you change either the source or the copy, you may also cause the other object to change too. That behavior contrasts with the behavior of a deep copy, in which the source and copy are completely independent.| developer.mozilla.org
