Arrays and loops

Store collections, iterate with for...of and accumulate results without duplicating statements.

About 10 minutes · Beginner

Read an ordered collection

An array stores values in order. Indexes start at zero, so scores[0] reads the first score. length gives the number of elements.

for...of visits values directly. Use it when you need each value rather than its property name; for...in serves a different purpose.

const scores = [60, 75, 81];
let total = 0;
for (const score of scores) {
  total += score;
}
console.log(total);

Expected output

A traditional for loop has initialization, a condition and an update. The condition is checked before every iteration.

Use index < array.length, not <=, to avoid reading one element past the end. Accessing that missing element returns undefined.

const topics = ["Values", "Conditions", "Loops"];
for (let i = 0; i < topics.length; i += 1) {
  console.log(`${i + 1}. ${topics[i]}`);
}

Expected output

1. Values
2. Conditions
3. Loops

filter creates a new array containing values for which the callback returns a truthy result. It does not remove values from the original array.

The arrow function below receives one score and returns the comparison result. We will build named reusable functions next.

const scores = [35, 62, 40, 28];
const passed = scores.filter(score => score >= 40);
console.log(passed.join(", "));

Expected output

62, 40

Count how many values in [2, 7, 4, 9] are even. A value is even when value % 2 === 0.

Show a possible solution

const values = [2, 7, 4, 9];
console.log(values.filter(value => value % 2 === 0).length); // 2

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