Lesson 7: Array Methods

Learning Goals

• Use map to transform values.
• Use filter to keep some values.
• Use find to get first matching value.
• Explain clearly what each array method returns.

Example

// Save as lesson7.js
let prices = [8000, 12000, 25000, 6000];

let expensive = prices.filter(function (p) {
  return p >= 10000;
});

let withTax = prices.map(function (p) {
  return p * 1.18;
});

console.log(expensive); // [12000, 25000]
console.log(withTax);   // [9440, 14160, 29500, 7080]

When we write this, we mean:

• filter keeps values that pass a test.
• map creates a new list with changed values.
• Original prices stays unchanged unless you overwrite it.

Deeper Explanation

Array methods help you think in transformations instead of manual loops. In real projects, this makes your code easier to read and easier to test. Someone can quickly understand your intention: keep some values, modify some values, or pick one matching value.

The key beginner rule is this: every method has a job. Use the method that matches your intention. If you want fewer items, use filter. If you want changed items, use map. If you want one item, use find.

Common Beginner Mistakes

1. Expecting filter to return one value instead of an array.
2. Forgetting to return inside callback functions.
3. Confusing map with forEach when a new array is needed.

Practice

1. Use filter to keep scores above 50.
2. Use map to add 5 marks to each score.
3. Use find to get first score below 40.

Homework

Given product prices, create one list for cheap items and one list with VAT added.

let productPrices = [3500, 9000, 12000, 5000];

let cheapItems = productPrices.filter(function (p) {
  return p <= 5000;
});

let pricesWithVat = productPrices.map(function (p) {
  return p * 1.18;
});

console.log(cheapItems);
console.log(pricesWithVat);