Rest Parameters and Spread Operators in Javascript

Javascript has a functionality where depending on where it's used, ... can either be a rest parameter or a spread operator.

Spread Operators

spread operators allow an iterable object like an array to be expanded, unpacking each element from the array.

let fourToSix = ['four', 'five', 'six']
let numbers = ['one', 'two', 'three', ...fourToSix ]

=> ["one", "two", "three", "four", "five", "six"]

Rest Parameters

A rest parameter will bundle up all remaining elements in a parameter into an array.

const greetings = (friend, ...everyoneElse ) => {
  console.log(`Hello, ${friend}!`)
  everyoneElse.forEach(person => console.log(`Nice to meet you, ${person}!`))
}

greetings('Lei', 'Cierra', 'Aiden', 'Pierre', 'Miranda')

=> Hello, Lei!
=> Nice to meet you, Cierra!
=> Nice to meet you, Aiden!
=> Nice to meet you, Pierre!
=> Nice to meet you, Miranda!