What is the spread operator used for in JavaScript?

The spread operator (ellipsis ...) is used for the following purposes:

1. Expanding elements of iterable objects (arrays, strings) into function arguments:

   const numbers = [1, 2, 3];
   
   function sum(a, b, c) {
     return a + b + c;
   }
   
   const result = sum(...numbers); // 1 + 2 + 3 = 6
   

2. Expanding elements of iterable objects (arrays, strings) into a new array:

   const array1 = [1, 2];
   const array2 = [3, 4];
   
   const combinedArray = [...array1, ...array2]; // [1, 2, 3, 4]
   

3. Copying an array by value:

   const originalArray = [1, 2, 3];
   const copiedArray = [...originalArray]; // Creates a new array with the same elements
   

4. Copying and merging object properties:

   const obj1 = { a: 1, b: 2 };
   const obj2 = { b: 3, c: 4 };
   
   const mergedObject = { ...obj1, ...obj2 }; // { a: 1, b: 3, c: 4 } - properties from obj2 overwrite those from obj1 with the same keys
   

5. Copying an object by value (shallow):

   const originalObject = { a: 1, b: { c: 2 } };
   const copiedObject = { ...originalObject }; // The object itself is copied, but nested objects are referenced
   

6. Expanding strings into arrays of characters:

   const str = "hello";
   
   const charArray = [...str]; // ["h", "e", "l", "l", "o"]