Could someone please explain this, and Rest and spread parameters better?

Question

function magic(...nums) {
let sum = 0;
nums.filter(n => n % 2 == 0).map(el => sum+= el);
return sum;
}
console.log(magic(1, 2, 3, 4, 5, 6));

This is REALLY confusing to me, help would be GREATLY appriciated. Thank you

Answer 1

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function magic(...nums){
}
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This is a rest parameter, and it is used to tell JS that this function has an unknown number of arguments. This is quite useful when working with dynamic data. You don't have a fixed number of arguments, and when you call the function, you are allowed to pass as many arguments as you want. The arguments that are placed into the function-call is then converted into an array. The "nums" variable is simply the name that the array of numbers is given when the function is called.

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nums.filter(n => n % 2 == 0).map(el => sum+= el);
-------------------------------------------------------------

The filter function filters numbers where the modulus is 0, meaning that the number divided by "2" gives 0 remainders. Example: 2 % 2 = 0 (because 2 divided by 2 is 0), 4 % 2 = 0 (because 4 divided by 2 is 2, and 2 divided by 2 is, again, 0). The map function then has an array of numbers which are dividable by 2, and then adds all the numbers together.

Your starting array:
[1, 2, 3, 4, 5, 6]

After filter:
[2, 4, 6]

2 + 4 + 6 = 12

Hope this helped! Feel free to message me if you have any further questions :)

Answer 2

All you have to do is to first remove the %2 =0 passes into the filter method.
Let's try breaking it
The argument ...nums in the magics are
1,2,3,4,5,6,
If we filter the nums aguments with %2= 0;
1%2 != 0,
2%2 = 0,
3%2 != 0,
4%2 = 0,
5%2 != 0,
6%2 = 0
Now from the filters above
Only 2 ,4,6 passed the test.
From the test passed item, we attached another method map: which says to add the whole item that passed the test to variable sum
Now sum = 2+4+6,
Which means sum = 12

So our answer is 12..
--credit to igwe
Big thanks to him!
and big thanks to Roolin!

Answer 3

Bryan

filter()??? map()????

function filter(n => n%2==0) try to check all nums for values that will give remainder 0 if divided by 2
filter(n => n %2 == 0) checks the objects "n" in nums with modulus operation and add the object with true result "el" in sum while discarding any false results,

function map(el => sum +=el) works here to add up the filtered nums i.e 2,4,6

in simple terms here we go;

var nums = [1,2,3,4,5,6], n = nums, sum = 0, el = 0;

//function filter(n => n%2==0)

for(let i=0; i<n.length; i++){
if (n[i]%2==0)
{
//function map(el => sum +=el)

el = n[i]; // 2, 4, 6
sum += el; //2+4+6
}
}
console.log(sum); //12