Refactor accumulate with Functional Programming / closure

old way:

function accumulate(arr) {

let result = []

let sum = 10

for (let i = 0; i < arr.length; i++) {

sum += arr[i]

result.push(sum)

}

return result

}

in functional programming

1. no mutation - don't mutate data
2. pure function - no side effects
3. const 

so we need to use filter, map, concat, reduce, 

closure: 

a closure is a function that references variables in the outer scope from its inner scope.

function accumulate(arr) {

let total = 10

return (() =>

arr.map((num) => {

total += num

return total

})

)

}

console.log(sum([1,2,3])())

=> [ 11, 13, 16 ]