As an exercise I'm trying to convert a map example to use reduce:
const numbers = [1, 2, 3, 4];
numbers.map(function(a,b) {
return a+b;
});
// [1, 3, 5, 7]
my attempt for same example with reduce:
numbers.reduce(
(sum, elem) => {
sum.push(elem + elem); // how can one add the index of array to the number instead of doubling it?
return sum;
}, []);
// [2, 4, 6, 8], instead I want: [1, 3, 5, 7]
Your result with reduce. Pass the third parameter, which is the index
const numbers = [1, 2, 3, 4];
var arr = numbers.reduce(
(sum, elem, index) => {
sum.push(elem + index);
return sum;
}, []);
console.log(arr);But
Your map function works as you want only by accidently :).
map function accepts as parameters not the 2 sibling items, but an item and it's index.
See here. If you change the pre-last item, you will get an other behaviour. In your logic the last item must be 11,if it accepts 2 sibling items, but not it is still 7, because it accepts the item and it's index.
Example with changed data.
1 -> 1 + 0 (index) = 1
2 -> 2 + 1 (index) = 3
3 -> 3 + 2 (index) = 5
4 -> 4 + 3 (index) = 7
const numbers = [1, 2, 7, 4];
var arr = numbers.map(function(a,b) {
return a+b;
});
console.log(arr);Output
1 -> 1 + 0 (index) = 1
2 -> 2 + 1 (index) = 3
7 -> 7 + 2 (index) = 9
4 -> 4 + 3 (index) = 7
reduce function accepts in it's basic form two parameters, the item of the array and the returned value from the previous iteration.
So I think you understood these functions not well.
You can read more in the Array#map() and Array#reduce()
You could add the previous item with a check for truthyness and the actual item. Return the array with concatinated value.
var numbers = [1, 2, 3, 4],
result = numbers.reduce((r, a, i, aa) => r.concat((aa[i - 1] || 0) + a), []);
console.log(result);
