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

Summary:

In javascript the ... is overloaded. It performs a different operations based on where the operator is used:

1. When used in function arguments of a function declaration/expression it will convert the remaining arguments into an array. This variant is called the Rest parameters syntax.
2. In other cases it will spread out the values of an iterable in places where zero or more arguments (function calls) or elements (array literals) are expected. This variant is called the Spread syntax.

Example:

Rest parameter syntax:

function rest(first, second, ...remainder) {
  console.log(remainder);
}

// 3, 4 ,5 are the remaining parameters and will be 
// merged together in to an array called remainder 
rest(1, 2, 3, 4, 5);

Spread syntax:

function sum(x, y, z) {
  return x + y + z;
}

const numbers = [1, 2, 3];

// the numbers array will be spread over the 
// x y z parameters in the sum function
console.log(sum(...numbers));


// the numbers array is spread out in the array literal
// before the elements 4 and 5 are added
const newNumbers = [...numbers, 4, 5];

console.log(newNumbers);