Reduce (javascript) | Chinese Rule of Eight

1	const numbers = [1,2,3];

Reduce sums up all the items in the array where the first parameter is the callback and second param is the starting value.

const result = numbers.reduce(

(sum, item) => sum + item,

0 // starting value

);

const grades = [60, 55, 80, 90, 99, 92, 75, 72];

const total = grades.reduce(sum);

function sum(total, grade) {

return total + grade;

}

Let’s group grades into an object:

const letterGradeCount = grades.reduce(groupByGrade, {});

function groupByGrade(resObj, grade) {

const { a = 0, b = 0, c = 0, d = 0, f = 0 } = resObj;

if (grade >= 90) {

return {...resObj, a: a + 1}

} else if (grade >= 80) {

return {...resObj, b: b + 1}

} else if (grade >=70) {

return {...resObj, c: c + 1}

} else if (grade >= 60) {

return {...resObj, d: d + 1}

} else {

return {...resObj, f: f + 1}

}