when to use reduce and reduceRight?

The order for reduce is from left to right, and it’s from right to left for reduceRight, as the following piece of code shows:

var arr = ["1", "2", "3", "4", "5"];

total1 = arr.reduce(function(prev, cur) {
    return prev + cur;
});

total2 = arr.reduceRight(function(prev, cur) {
    return prev + cur;
});

console.log(total1); // => 12345
console.log(total2); // => 54321