Med Java

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====== Rest vs Spread operator ======

The **rest operator (...)** and the **spread operator (...)** in JavaScript are written the same way but are used for two different purposes. \\ 
Their usage depends on the context, and they perform opposite operations.

<note important>
**Rest Operator**: Collects multiple elements into an array. \\ 
**Spread Operator**: Spreads the elements of an iterable out into individual elements.
</note>

===== Rest operator (...) =====

The rest operator is used to **collect all remaining iterable elements into an array or object**. 

==== Usage in function arguments ====

This can be particularly useful in function signatures. \\ 

Example:
<code javascript>
function sum(...numbers) {
  return numbers.reduce((total, num) => total + num, 0);
}

console.log(sum(1, 2, 3, 4, 5)); // Output: 15
</code>
Here, the **...numbers** argument takes all passed numbers and puts them into an **array** called **numbers**.

<note important>
Rest function parameter should be the last one in the list:
<code javascript>
function validFunction(param1, param2, ...rest) {
  console.log(param1, param2, rest);
}
</code>
</note>

<note important>
A function cannot have multiple rest parameters.
</note>

==== Destructuring Arrays ====

You can use the rest operator to collect the rest of the elements into an array when destructuring:

<code javascript>
const [first, second, ...rest] = [1, 2, 3, 4, 5];
console.log(rest); // Output: [3, 4, 5]
</code>

==== Destructuring Objects ====

Similarly, when destructuring objects, the rest operator can collect the remaining own enumerable property keys into a new object:

<code javascript>
const {a, b, ...rest} = {a: 1, b: 2, c: 3, d: 4};
console.log(rest); // Output: {c: 3, d: 4}
</code>

==== Clone Objects and Arrays ====

It's a handy way to create a shallow copy of an object or an array.

<code javascript>
const obj = {a: 1, b: 2};
const clonedObj = {...obj}; // Output: {a: 1, b: 2}
</code>

===== Spread operator (...) =====

The spread operator is used to **expand elements of an iterable (such as an array, object, or string)** where zero or more arguments (for function calls), elements (for array literals), or key-value pairs (for object literals) are expected. \\ 

==== Example with Arrays ====

<code javascript>
const arr1 = [1, 2, 3];
const arr2 = [...arr1, 4, 5];
console.log(arr2); // Output: [1, 2, 3, 4, 5]
</code>

Here, the **...arr1** spreads out the elements of **arr1**, so they become individual elements in **arr2**.

==== Example with Objects ====

<code javascript>
const obj1 = { a: 1, b: 2 };
const obj2 = { ...obj1, c: 3 };
console.log(obj2); // Output: { a: 1, b: 2, c: 3 }
</code>

Here, the **...obj1** spreads out the properties of **obj1**, so they become individual properties in **obj2**.