1const array1 = [1, 2, 3, 4];
2const reducer = (accumulator, currentValue) => accumulator + currentValue;
3
4// 1 + 2 + 3 + 4
5console.log(array1.reduce(reducer));
6// expected output: 10
7
8// 5 + 1 + 2 + 3 + 4
9console.log(array1.reduce(reducer, 5));
10// expected output: 15