What are Default Parameters?

Default Parameters is a new way of defining default variables in JavaScript it is available in the ES6 or ECMAScript 2015 Version.

//ES5 Version
function add(a,b){
  a = a || 0;
  b = b || 0;
  return a + b;
}

//ES6 Version
function add(a = 0, b = 0){
  return a + b;
}
//If we don't pass any argument for 'a' or 'b' then 
// it's gonna use the "default parameter" value which is 0
add(1); // returns 1

We can also use Destructuring in Default Paremeters.

function getFirst([first, ...rest] = [0, 1]) {
  return first;
}

getFirst();  // returns 0
getFirst([10,20,30]);  // returns 10

function getArr({ nums } = { nums: [1, 2, 3, 4] }){
    return nums;
}

getArr(); // returns [1, 2, 3, 4]
getArr({nums:[5,4,3,2,1]}); // returns [5,4,3,2,1]

We can also use the parameters defined first to the parameters defined after them.

function doSomethingWithValue(value = "Hello World", callback = () => { console.log(value) }) {
  callback();
}
doSomethingWithValue(); //logs "Hello World"