Array and Object Iteration

Iteration methods process every element without writing manual loops. They make transformations, filtering, and aggregation easier to express—and they’re the building blocks for clean data pipelines. Today you’ll meet the four essentials: forEach, map, filter, and reduce.

Step 1: Meet the Methods

• forEach runs a callback for every element—great for side effects like logging.
• map transforms each element and returns a new array.
• filter keeps only elements that pass a test.
• reduce combines elements into a single value (number, object, string, etc.).

Each method receives a callback with (value, index, array) plus optional context arguments.

Step 2: forEach for Side Effects

const learners = ["Ada", "Grace", "Kayra"];

learners.forEach((name, index) => {
  console.log(`${index + 1}. ${name}`);
});

• Executes the callback for each item.
• Returns undefined; use it when you’re not creating a new array.
• If you need to break early, switch to a for loop instead.

Step 3: map for Transformation

const scores = [10, 12, 15];

const doubled = scores.map((score) => score * 2);
console.log(doubled); // [20, 24, 30]

• Returns a new array with the same length.
• Leave the original untouched.

Step 4: filter for Selection

const lessons = [
  { title: "Arrays", duration: 20 },
  { title: "Objects", duration: 25 },
  { title: "Iteration", duration: 20 }
];

const longLessons = lessons.filter((lesson) => lesson.duration > 20);
console.log(longLessons);
// [{ title: "Objects", duration: 25 }]

• Returns a new array with only the items that return true.
• Output length may be shorter (or even 0).

Step 5: reduce for Aggregation

const durations = [20, 25, 20];

const total = durations.reduce((sum, minutes) => sum + minutes, 0);
console.log(total); // 65

• Takes an accumulator (sum) and current value (minutes) plus an initial value (0).
• Use it for totals, grouping, composing new objects, and more.

Step 6: Chaining Methods

Combine methods in sequence.

const learners = [
  { name: "Ada", progress: 0.9 },
  { name: "Grace", progress: 0.6 },
  { name: "Kayra", progress: 0.8 }
];

const ahead = learners
  .filter((learner) => learner.progress >= 0.8)
  .map((learner) => learner.name.toUpperCase());

console.log(ahead); // ["ADA", "KAYRA"]

• Each method returns a new array, so the next method can keep refining it.

Step 7: Iterating Objects

Objects don’t have these methods, but you can convert them to arrays.

const stats = { day: 5, completed: 12, remaining: 3 };

Object.entries(stats).forEach(([key, value]) => {
  console.log(`${key}: ${value}`);
});

• Object.keys, Object.values, and Object.entries return arrays you can iterate.

Step 8: Choosing the Right Tool

• Use map if you’re transforming every element.
• Use filter if you’re excluding some elements.
• Use reduce if you’re combining into a single result.
• Stick with forEach for side effects or when return values aren’t needed.

Step 9: Practice Prompts

1. Use forEach to log "Lesson: <title>" for each item in const lessons = ["Intro", "Arrays", "Objects"];.
2. Build a new array of durations using map on const sessions = [15, 20, 30]; that adds 5 minutes buffer to each.
3. Filter const scores = [45, 72, 88, 95]; to keep only scores >= 70.
4. Sum every number in const progress = [0.1, 0.3, 0.6]; with reduce.
5. Convert const course = { title: "Arrays", level: "beginner" }; into an array of "key: value" strings using Object.entries and map.

Key Takeaways

• ✅ Iteration methods reduce boilerplate and clarify intent.
• ✅ map, filter, and reduce create new data structures without mutating originals.
• ✅ forEach is best for running side effects on each element.
• ✅ Convert objects to arrays with Object.entries to reuse array helpers.

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🎯 Quick Check

1. When would you prefer map over forEach?
2. What initial value should you pass to reduce when summing numbers?
3. How can you iterate key/value pairs inside an object?
4. Why are chaining methods powerful for data pipelines?

Next up: tie arrays and objects together in a mini project that tracks todos. ✅