Summary - Array methods

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.

Exercise 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.

Show example 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);

Exercise 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.

Show example 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);

Test yourself

Match each method to its purpose

map

Transform
filter

Filter
reduce

Fold to one value
find

First match
forEach

Side effect
Which chain is correct?
- reduce().map().filter()
- map().reduce().filter()
- forEach().map().filter()
- filter().map().reduce()
Show answer

filter().map().reduce() — first filter, then map, finally reduce.

Chained call

const nums = [1, 2, 3, 4];
const result = nums.filter(n => n > 1).____(n => n * 2);

Show answer

const nums = [1, 2, 3, 4];
const result = nums.filter(n => n > 1).map(n => n * 2);

Next lesson: Object literal

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