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

ES6 has new feature three dots ...

Here is how we can use these dots:

1. As Rest/Collector/Gather

var [c, ...m] = [1,2,3,4,5]; // m -> [2,3,4,5]

Here ...m is a collector, it collects the rest of the parameters. Internally when we write:

var [c, ...m] = [1,2,3,4,5]; JavaScript does following

var c = 1,
    m = [2, 3, 4, 5];

2. As Spread

var params = [ "hello", true, 7 ];
var other = [ 1, 2, ...params ]; // other => [1,2,"hello", true, 7]

Here, ...params spreads so as to adding all of its elements to other

Internally JavaScript does following

var other = [1, 2].concat(params);

Hope this helps.

Answer 2

Added in ES6 these three dots ... has two meanings, Spread operator and Rest parameter

Spread operator: You use the three dots to expand iterables, by iterables I mean arrays, string, etc. As arguments. For example Math.max() function expect an indeterminate number of arguments so you can use Spread operator to expand elements as arguments on Math.max() function. Here an example from mdn

console.log(Math.max(1, 3, 2));
// expected output: 3

console.log(Math.max(-1, -3, -2));
// expected output: -1

var array1 = [1, 3, 2];

console.log(Math.max(...array1));
// expected output: 3

Another use case is to add, for example having this array

const videoGames = ['mario galaxy', 'zelda wind waker', 'ico'];

You can add it to another array

const favoritesVideoGames = ['Shadow of the colosus', ...videoGames];

Then favoritesVideoGames value is

[ 'Shadow of the colosus', 'mario galaxy', 'zelda wind waker', 'ico' ]

About Rest parameter, here the MDN definition

The rest parameter syntax allows us to represent an indefinite number of arguments as an array.

This means you can pack many elements into a single element

Here an example from MDN

function sum(...theArgs) {
  return theArgs.reduce((previous, current) => {
    return previous + current;
  });
}

console.log(sum(1, 2, 3));
// expected output: 6

console.log(sum(1, 2, 3, 4));
// expected output: 10

I usually get confused with these three points, this illustration by @stephaniecodes helps me to remember its logic. I mention that I took inspiration from this illustration to answer this question.

I hope it is useful.

Answer 3

considering 3 scenarios

1] without using any operator

function add(x, y) {
  return x + y;
}

add(1, 2, 3, 4, 5) // returns 3  (function will takes first 2 arg only)

2] with rest operator

function add(...args) {
  let result = 0;

  for (let arg of args) result += arg;

  return result
}

add(1) // returns 1
add(1,2) // returns 3
add(1, 2, 3, 4, 5) // returns 15

- we can gather any number of arguments into an array

3] with spread operator

const arr = ["Joy", "Wangari", "Warugu"];
const newArr = ["joykare", ...arr];

The value of newArr will be [ 'joykare', 'Joy', 'Wangari', 'Warugu' ]

another one

function add(a, b, c) {
  return a + b + c ;
}
const args = [1, 2, 3];

add(...args);

-We have been using arrays to demonstrate the spread operator, 
but any iterable also works. So, if we had a 
string const str = 'joykare', [...str] translates to [ 'j', 'o', 'y', 'k', 'a', 'r', 'e' ]

Answer 4

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);

Answer 5

Basically like in Python:

>>> def func(first, *others):
...    return [first, *others]
>>> func('a', 'b', 'c')
['a', 'b', 'c']

Answer 6

In reference to this i cant understand how we are passing a function and returning arguments in javascript

Function is a set of instructions that takes some input and processes them and returns result.

here we have an array [1, 2, 3, 4, 5, 6], and filter function iterates over each element and passes each element to positive functions which returns the number if it is even, else skips it.

trace:

1 => Filter(1) => positive(1) => skips 1,
2 => Filter(2) => positive(2) => returns 2,
3 => Filter(3) => positive(3) => skips 3,
...
6 => Filter(6) => positive(6) => returns 6

hence the result [2, 4, 6]