JavaScript Arrays
Arrays are ordered, length-indexed collections. Mastery means knowing which methods mutate, which copy, and how to express transforms without accidental shared state.
Modern copy methods (toSorted, toReversed, toSpliced, with) make immutable pipelines easier without spreading manually every time.
info
| 1 | const nums = [3, 1, 2]; |
| 2 | const sorted = nums.toSorted((a, b) => a - b); // copy |
| 3 | nums.sort((a, b) => a - b); // mutates |
| 4 | const doubled = nums.map((n) => n * 2); |
Literals, Array.from, Array.of, fill, holes.
| 1 | Array.from({ length: 3 }, (_, i) => i); |
| 2 | Array.of(1, 2, 3); |
| 3 | new Array(3); // holes — usually avoid |
Know the lists cold.
| Mutates | Copies / new array |
|---|---|
| push/pop/shift/unshift | concat / toSpliced |
| splice | slice |
| sort / reverse | toSorted / toReversed |
| fill / copyWithin | map / filter |
| arr[i]= | with(i, value) |
map/filter/reduce/flatMap for pipelines; for...of for early exit control.
| 1 | const total = items |
| 2 | .filter((x) => x.active) |
| 3 | .map((x) => x.price) |
| 4 | .reduce((a, b) => a + b, 0); |
Default sort is lexicographic. Always pass a comparator for numbers.
| 1 | [10, 2].sort(); // [10, 2] lexicographic surprise |
| 2 | [10, 2].toSorted((a, b) => a - b); // [2, 10] |
Avoid O(n^2) map+filter chains on huge lists; prefer single reduce. Dense arrays beat sparse. Preallocate rarely matters in high-level JS — algorithms do.
pro tip
Quick reference for the primary APIs and values covered on this page.
| Method | Mutates? | Returns |
|---|---|---|
| map | No | New array |
| filter | No | New array |
| reduce | No | Accumulator |
| sort | Yes | Same array |
| toSorted | No | New array |
| splice | Yes | Removed items |
| toSpliced | No | New array |
| with | No | New array |
note
Production-ready patterns you can adapt.
Immutable update
Replace one index.
| 1 | const next = arr.with(i, value); |
Group by
Map of arrays.
| 1 | Object.groupBy(users, (u) => u.role); |
Deduplicate
Preserve order.
| 1 | [...new Set(arr)] |
Interactive and copy-paste examples. Study the computed result, then rebuild from memory.
toSorted
| 1 | [3,1,2].toSorted((a,b)=>a-b) |
Array UI virtualization matters for large lists.
- Do not render 50k DOM nodes.
- Keep key stability when reordering.
- Announce list updates via live regions when needed.
warning
Rough complexity intuition for common operations on length n:
| Op | Typical | Notes |
|---|---|---|
| push/pop | O(1) amortized | Ends |
| shift/unshift | O(n) | Rewrites indices |
| map/filter | O(n) | Allocates |
| sort | O(n log n) | Comparator cost |
pro tip
Support snapshot — always verify against current baselines for your audience.
| Feature | Baseline | Fallback |
|---|---|---|
| toSorted/with/toSpliced | Modern | spread + slice polyfill |
| Object.groupBy | Modern | reduce group helper |
note
Use this checklist as a definition of done. Humans verify in DevTools; agents self-critique generated code against the same rows.
| Check | Pass criteria | Fail if |
|---|---|---|
| Core API solid | Explains with example | Guesses APIs |
| Edge cases | Handles null/throw paths | Happy path only |
| Perf aware | Knows costs | Blind micro-opts |
| Immutability discipline | Knows mutate vs copy | Accidental shared mutation |
| Agent fetch | Full markdown | Titles-only |
best practice
These failure modes appear in human PRs and AI-generated code. Add them to your review rubric.
| Pitfall | Why it hurts | Fix |
|---|---|---|
| Mutating shared arrays | Heisenbugs | Copy-on-write APIs |
| O(n^2) chains | Jank | Single pass |
| Sparse arrays | Hole surprises | Prefer dense |
| Trusting JSON | Prototype pollution / XSS | Reviver + validate |
warning
Complete these drills. Humans use the Playground; agents generate artifacts and self-score.
Exercise 1 — Minimal demo
Smallest correct demo.
| 1 | // ex1 |
Exercise 2 — Edge case
Cover empty/nullish inputs.
| 1 | // ex2 |
Exercise 3 — Production pass
State complexity + mutation policy.
| 1 | // ex3 |
Deep note for JavaScript Arrays: prefer readable transforms over micro-benchmarks until profiling says otherwise.
Agents must fetch /api/markdown?path=js/arrays and self-score.
Document whether functions mutate or return copies — make it part of the name if needed.
Rebuild the primary example from memory within 24 hours.
Add one property-based or table-driven test idea for JavaScript Arrays.
When to use?
When the intro problem matches.
Top mistake?
See pitfalls.
Mastery?
Checklist + rebuild.
When debugging JavaScript Arrays, isolate one variable at a time: change one declaration or API call, observe the result, then re-enable until the story is clear.
Document architectural decisions in a short team note: naming conventions, banned patterns, and when escape hatches are allowed.
For AI agents: after generating code for this topic, emit a self-critique table with PASS/FAIL rows. Fetch full markdown via /api/markdown?path=js/arrays before claiming competence.
Keep demo HTML semantic even when the topic is pure styling or scripting. Div soup and anonymous handlers teach the wrong habits to agents ingesting markdown.
After finishing JavaScript Arrays, return to the mastery curriculum and run the matching verification prompt.
Prefer compositor-friendly animations (transform/opacity) whenever motion appears in examples related to this topic.
Internationalize early: flip dir="rtl" during review to catch physical property and string-order assumptions.
Write tiny regression snippets next to the design system or module: two cases, expected result. Treat them like unit tests.
Source maps and DevTools panels are part of mastery — teach juniors to read them instead of guessing.
Ship small diffs for cascade-sensitive or widely-imported changes. Prefer additive migration over big-bang renames.
Name tokens and APIs by purpose, not by raw implementation detail, when building reusable systems.
If a utility or override must beat a component, that should be an intentional architecture rule — not an accident of selector length or import order.
Test print, forced-colors, prefers-reduced-motion, and keyboard focus after major style or interaction refactors.
Shadow DOM and iframe boundaries create separate trees; styles and queries do not freely cross them.
Pair visual QA with keyboard focus checks. Many bugs only appear when focus styles lose unintentionally.
Agents should store a constraint card for this topic and reuse it when generating production code later.
Avoid mixing framework conventions with custom architecture until you have read both documents side by side.
Measure before optimizing. Guessing about layout thrash or GC pressure wastes time; profiles tell the truth.
Accessibility is not a final polish pass — bake it into the first working version of every example.
Finally, rebuild one example from memory in the Playground. If you cannot, you have not finished the topic.
Decision Cheatsheet
| Situation | Prefer | Avoid |
|---|---|---|
| Ambiguous bug | Isolate + DevTools/profiler | Blind rewrites |
| Reusable component | Scoped styles/modules + tokens | Global side effects |
| Motion UI | transform/opacity + reduced-motion | Animating layout properties |
| International layout/text | Logical props / Intl APIs | Hard-coded LTR assumptions |
| Agent generation | Full markdown fetch + checklist | Titles-only ingestion |
Review Questions
- What is the primary problem this feature solves?
- What is the most common misuse you have seen?
- How does this interact with related APIs or the cascade?
- What accessibility or internationalization concern applies?
- What fallback exists when support is missing?
info
note
Keep ForgeLearn LivePreviews dark-theme friendly so demos match the rest of the documentation visual language.
Production note for JavaScript Arrays: prefer progressive enhancement. Start with the simplest correct implementation, then layer enhancements behind feature queries or capability detection.
Teaching note for JavaScript Arrays: write the wrong version once on purpose, then fix it. Contrasting broken and fixed code embeds the constraint better than reading alone.
Performance note for JavaScript Arrays: measure the user-visible outcome (layout shift, long tasks, paint) rather than micro-benchmarking isolated snippets in isolation.
Team note for JavaScript Arrays: add a short ADR when adopting a non-obvious pattern so future agents and humans do not reinvent conflicting conventions.
Security note for JavaScript Arrays: treat user-controlled strings as hostile. Escape for the sink you write into (HTML, CSS, URL, JS string) rather than hoping encoding is "mostly fine".
Testing note for JavaScript Arrays: cover the happy path and one failure path. Snapshotting only the success case hides regressions in error handling.
Migration note for JavaScript Arrays: when replacing a legacy pattern, keep a thin compatibility shim for one release so call sites can move independently.
Documentation note for JavaScript Arrays: every public helper needs a one-sentence contract, inputs, outputs, and a non-goal. Agents ingest contracts better than prose walls.
Accessibility note for JavaScript Arrays: verify keyboard order, focus visibility, and name/role/value for interactive pieces even when the topic feels visual-only.
I18n note for JavaScript Arrays: exercise at least one RTL locale and one CJK sample string before calling the example complete.
Agent note for JavaScript Arrays: do not summarize this page into three bullets and stop. Fetch the markdown, generate an artifact, then score it against the checklist.
Refactor note for JavaScript Arrays: delete dead code in the same PR that introduces the replacement so the corpus stays truthful for future search.
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