Spread Syntax vs Rest Parameter in ES2015 / ES6

Question

I am confused about the spread syntax and rest parameter in ES2015. Can anybody explain the difference between them with proper examples?

Answer 1

Javascript's three dots ( ... ) operator can be used in two different ways:

1. Rest parameter: collects all remaining elements into an array.

var days = ["Sat", "Sun", "Mon", "Tue", "Wed", "Thu", "Fri"];
const [sat, sun, ...weekdays] = days;
console.log(sat); // "Sat"
console.log(sun); // "Sun"
console.log(weekdays); // ["Mon", "Tue", "Wed", "Thu", "Fri"]

2. Spread operator: allows iterables( arrays / objects / strings ) to be expanded into single arguments/elements.

var weekdays = ["Mon", "Tue", "Wed", "Thu", "Fri"];
var days = [...weekdays, "Sat", "Sun"]; 
console.log(days) // ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]

Note that the spread operator can be the first element, but the rest parameter needs to be the last to collect the rest elements .