Rest Operator In JavaScript
The rest operator actually uses the same syntax as the spread operator: .... It’s usage determines whether you’re using it as the spread or rest operator.
Refresher on the Spread operator
The spread operator allows you to pull elements out of an array (split the array into a list of its elements) or pull the properties and values out of an object.
Here's the spread operator used on an array:
const oldArray = [1, 2, 3];
const newArray = [...oldArray, 4, 5]; // [1, 2, 3, 4, 5];
Here's the spread operator used on an object:
const oldObject = {name: 'Daniel'};
const newObject = {...oldObject, age: 22}; // {name: 'Daniel', age: 22}
The spread operator is useful for cloning arrays and objects. Since both are stored by reference (a reference is created to the object/array in memory), copying them safely (i.e. not making another reference) can be tricky. With the spread operator you have an easy way of creating a clone of the object or array.
Using the Rest Operator
The Rest operator allows you to have an almost unlimited amount of arguments as an array.
Here's the rest operator used to turn arguments into an array:
function addAllNumbers(...numbers) {
return numbers.reduce(function (previousNumber, currentNumber) {
return previousNumber + currentNumber;
});
}
console.log(addAllNumbers(1, 2, 3, 4, 5)); // 15
Conclusion
Some might argure that the rest operator is not as useful as the spread operator, but it provides a nice way to turn arguments of a function into an array.