Arrays and common methods

Arrays preserve order and provide methods for expressing common collection operations.

What you will learn

Create and index an array
Transform values with map
Filter items with a predicate

An ordered collection

An array stores values in order and uses zero-based indexes. Read a value with brackets, add to the end with push, and check length before relying on a position. Arrays can contain objects and other arrays, but consistent item shapes are easier to process.

Transform with map

map calls a function for each item and returns a new array containing the results. It is useful when every item should become a corresponding output, such as turning scores into labels. The original array remains unchanged unless the callback mutates an item it references.

Select with filter

filter returns a new array containing items for which its callback returns a truthy result. The callback should express a simple yes-or-no rule. find is useful when the task needs the first matching item rather than a new collection of all matches.

Choose a method by intent

Use a loop when the task involves several coordinated steps or early exits. Use map, filter, or find when the method name closely describes the operation. Clear intent helps a reader distinguish transformation from selection without tracing every line.

Sort numbers with an explicit comparator

The default sort operation converts values to strings, so a numeric array may not end up in numeric order. Supply a comparison function when sorting numbers, and remember that sort mutates the array. If the original order matters, make a copy before sorting and test the result with a single-digit and a multi-digit value.

const minutes = [10, 25, 40];
const longSessions = minutes.filter((value) => value >= 25);
const labels = longSessions.map((value) => `${value} min`);
console.log(labels.join(', '));

Expected output

25 min, 40 min

Practice exercise

Given an array of scores, create a new array containing only passing scores.

Show a hint

Use filter with a comparison callback.

Show one possible solution

const passing = scores.filter((score) => score >= 40); keeps scores at or above the chosen threshold.

Quick check

What does map return? A new array with each item transformed The first matching item

Frequently asked questions

Does filter change the original array?

<p>filter creates a new array, but if its items are objects, both arrays can still refer to the same objects. Avoid mutating shared objects unexpectedly.</p>