Summary - Array methods
- Node.js course
- Beginner
- Lesson 64/116
- 5 min
Combine map, filter and reduce to write a sales report.
Summary - Array methods
In this group you learned five array methods. map transforms each element, filter keeps the matching ones, reduce folds to a single value, find returns the first match, and forEach walks each element for side effects.
You can chain them: prices.filter(p => p > 100).map(p => p * 1.1) first keeps the expensive ones, then adds tax. Chaining is a powerful way to process data step by step.
In the final task you will build a sales report: filter sales above 100, apply a 10% commission to each, then sum the commissions. You will use all three methods together.
Practice Sales report
Define an array of sales [80, 150, 200, 60]. Use filter to keep sales above 100, map to compute the 10% commission of each (sale * 10 / 100), then reduce to sum the commissions (initial 0). Print the total with console.log. You can chain the calls.
Starter code
const sales = [80, 150, 200, 60];
// Filter above 100, apply 10% commission, sum and printSample solution
const sales = [80, 150, 200, 60];
const total = sales.filter(s => s > 100).map(s => s * 10 / 100).reduce((sum, c) => sum + c, 0);
console.log(total);Expected output
35
Practice Total and average
Define an array of scores [40, 60, 80, 20]. Use reduce to compute the sum (initial 0) and print it with console.log. Then compute the average by dividing by the number of scores and print that too.
Starter code
const scores = [40, 60, 80, 20];
// Compute the total and the average, print bothSample solution
const scores = [40, 60, 80, 20];
const total = scores.reduce((sum, s) => sum + s, 0);
console.log(total);
const avg = total / scores.length;
console.log(avg);Expected output
200 50
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match each method to its purpose
| Term | Meaning |
|---|---|
map |
Transform |
filter |
Filter |
reduce |
Fold to one value |
find |
First match |
forEach |
Side effect |
2 Which chain is correct?
- reduce().map().filter()
- map().reduce().filter()
- forEach().map().filter()
- filter().map().reduce() Correct answer
Explanation: first filter, then map, finally reduce.
3 Chained call
const nums = [1, 2, 3, 4];
const result = nums.filter(n => n > 1).____(n => n * 2);const nums = [1, 2, 3, 4];
const result = nums.filter(n => n > 1).map(n => n * 2);Explanation: map doubles each remaining element.