restspread

restspread is a JavaScript feature introduced in ECMAScript 2018 that allows you to expand an iterable (like an array) or an object into its individual elements or properties. It serves two primary purposes: the rest parameter and the spread syntax.

The rest parameter, used in function declarations, collects an indefinite number of arguments into a single array. For example, a function like `function sum(...numbers)` would collect all arguments passed to `sum` into an array named `numbers`. This is useful when you don't know how many arguments a function will receive beforehand.

The spread syntax, on the other hand, is used in contexts where multiple elements or properties are expected. When used with arrays, it expands an array into individual elements. For instance, `const arr1 = [1, 2]; const arr2 = [...arr1, 3, 4];` would result in `arr2` being `[1, 2, 3, 4]`. It can also be used to create shallow copies of arrays or to merge arrays.

When applied to objects, the spread syntax allows you to copy the enumerable own properties of an object into a new object. For example, `const obj1 = { a: 1, b: 2 }; const obj2 = { ...obj1, c: 3 };` would create `obj2` as `{ a: 1, b: 2, c: 3 }`. This is particularly helpful for merging objects or creating modified copies of existing objects without mutating the original. It's important to note that object spread creates shallow copies, meaning nested objects are still referenced, not duplicated.