Introduction to JavaScript rest operator. To demonstrate the rest operator, I need to begin with a function. Function sumAll. And for the parameter, three dots, followed by the abbreviation args, short for arguments. Now, inside of the function, I'm gonna console log the arguments. Down below, I will call the function passing in the arguments of one, two, three, four, and five. I will execute the function and in the console, there is an array, and each one of the array's elements is one of the arguments that were passed through. The spread operator is designed to separate elements from an iterable to spread them apart. The rest operator allows you to take any remaining arguments, the rest of them, and place them into a data collection, in this case into an array.

Taking this a little further, I'm gonna add two parameters before the arguments parameter, X and a Y. I will execute the function again. And in the console, we can see that the arguments array only has the values of three, four, and five, capturing the remaining arguments. The rest of them. Now there is one specific rule. The rest parameter has to be the final parameter. I'm gonna move the Y parameter to be after the arguments parameter. When I execute this function, there is an error in the console stating that the rest parameter must be the last formal parameter. Now the rest operator isn't solely designed for function parameters. It could also be part of the destructuring of an object or an array.

On the screen, I have a user object in the numbers array. I will begin by destructuring the user object. Const, curly brackets, username, which is the direct property that I am destructuring. Now the rest syntax. Three dots, and the object declaration, userInfo. And I will set this equal to the user object. Now I will console log userInfo. I will execute this. And in the console, there's a new object with the email, first name, and last name properties from the user object. The rest operator is flexible when it comes to destructuring. When working with objects, whatever remaining properties that are not being destructured out of the object will be captured into a new object with the given name. And the same principles apply when destructuring an array. I'm going to comment out the console log of userInfo.

Now I will type const, square brackets. For the first element it will be assigned to a firstNumber declaration. And then for the remaining elements, I'm gonna use the rest syntax, and assign it to an array name remainingNumbers. And I will set this equal to the numbers array. Now down below, I'm going to console log the remainingNumbers array, and execute it. And in the console, there's an array with the values of two, three, four, and five. And that's it. Thanks for watching at Cloud Academy.