I need help to understand and practice the reduce() method in Js.
I want to calculate the total number of exercises in each course with the help of reduce(). \
Is this the correct way?
s = previousValue ,
p = currentValue, \
Update
as @Rani Sharim mentioned in the comment, I have to use reduce() on the parts array.
courses.map((course) => { console.log(course.parts.reduce((s, p) => s + p.exercises)) })
const courses = [
{
name: "Half Stack application development",
id: 1,
parts: [
{
name: "Fundamentals ",
exercises: 10,
id: 1,
},
{
name: "Using data",
exercises: 7,
id: 2,
},
{
name: "State ",
exercises: 14,
id: 3,
},
{
name: "React",
exercises: 11,
id: 4,
},
],
},
{
name: "Node.js",
id: 2,
parts: [
{
name: "Routing",
exercises: 3,
id: 1,
},
{
name: "Middlewares",
exercises: 7,
id: 2,
},
],
},
];
First argument usually is previous value, but You can initialize first argument using initialValue.
reduce((previousValue, currentValue, currentIndex, array) => { ... }, initialValue)
const courses = [
{
name: "Half Stack application development",
id: 1,
parts: [
{ name: "Fundamentals ", exercises: 10, id: 1, },
{ name: "Using data", exercises: 7, id: 2, },
{ name: "State ", exercises: 14, id: 3, },
{ name: "React", exercises: 11, id: 4, },
],
},
{
name: "Node.js",
id: 2,
parts: [
{ name: "Routing", exercises: 3, id: 1, },
{ name: "Middlewares", exercises: 7, id: 2, },
],
},
];
courses.forEach(course => {
course.totalExercises = course.parts.reduce(
(accumulator, part) => accumulator + part.exercises,
0 // initial value for accumulator
);
console.log(course.name, ', total exercises:', course.totalExercises);
});
console.log(courses);To get an idea how reduce works based on failure-success try to put console.log inside reducer:
const parts = [ {exercises: 5}, {exercises: 3}, {exercises: 7} ];
const result = parts.reduce(
(prev, curr, _, arr) => {
console.log('prev: ', prev);
console.log('curr: ', curr);
console.log('--------');
return prev + curr.exercises;
},
0
);
console.log('result:', result);