1. Input Array

Variable:

Presets:

[10, 20, 30, 40]

✓ Valid array · 4 items · all numbers

2. Method

Transform

⭐ Most Popular Methods

3. Callback

Style:

Presets:

Signature: (element, index, array) => value

n => n * 2

Ready

Code

Result

Success Array(4) mutated: false

Input Array

Output Result

View Raw Result JSON

Step-by-step Visualization

Step 0 / 0

Speed:

💡

Ready

Run an array method to step through evaluation.

1. Input Array 4 items

No input items yet

↓ passes item to callback

2. Callback Evaluation Idle

Press ▶ Play or Next ▶ to step through.

↓ determines output result

3. Output State Ready

Output builds as steps execute.

📌 What to remember about .map()

Trace Log (0 steps)

#	Args Preview	Returned	Type	Acc State	Status
No callback trace steps recorded.

[]

Engine Self-Test Suite (26 Verification Cases)

Validates native array methods, short-circuiting, sparse holes, parseInt gotchas, error capture, and 2-second timeout protection.

Tests Pending

1. Pipeline Input Array

Pipeline Source

Variable:

Presets:

[
  { id: 1, name: "Alice", age: 28, active: true },
  { id: 2, name: "Bob", age: 34, active: false },
  { id: 3, name: "Charlie", age: 22, active: true },
  { id: 4, name: "Diana", age: 31, active: true }
]

Valid array · 4 items · array of objects

Pipeline Stages

4 stages

⚡ Pipeline Performance & Trace

Total calls: 0

filter: 4 calls · map: 3 calls · sort: 2 calls

💡 Optimization Tip: Filtering before mapping is optimal: non-matching items are discarded before callback transformations run.

Generated Chained Code

Clean chained snippet with method calls

Final Pipeline Output

string

Pipeline completed successfully 0 ms

Beginner #1

Double every number

Given an array of numbers, return a new array where each number is doubled.

Input Array (arr):

[1, 2, 3, 4]

Expected Output:

[2, 4, 6, 8]

💡 Suggested Method: .map() (Hint, not strictly enforced)

Write Your Solution: Write callback or expression using arr

n => n * 2

Crucial Gotchas & JavaScript Traps

Common stumbling blocks that trip up developers of all experience levels.

⚠️ map() vs forEach() forEach() always returns undefined! It cannot be chained. Use map() when you want a new transformed array, and forEach() strictly for external side effects.

⚠️ filter() Truthiness Trap filter() evaluates callback returns by truthiness, not strict boolean true. Values like 0, "", null, and NaN evaluate to falsey and are discarded!

⚠️ ['1','2','3'].map(parseInt) Produces [1, NaN, NaN]. parseInt(string, radix) takes a radix! map passes (item, index), so parseInt('2', 1) evaluates with invalid base 1, yielding NaN. Use n => parseInt(n, 10) or Number.

⚠️ reduce() on Empty Array Calling [].reduce(...) without an initial value throws a runtime TypeError: Reduce of empty array with no initial value. Always provide an explicit initial accumulator!

⚠️ Default sort() is Lexicographic [10, 9].sort() returns [10, 9] because items convert to strings and "10" precedes "9". Plus, sort() mutates in-place! Use (a, b) => a - b or toSorted().

⚠️ Async Callbacks in map() arr.map(async x => ...) returns an array of Promises, not the resolved data. You must wrap with await Promise.all(...) to wait for them.

⚠️ Sparse Array Holes Skipped Empty holes in sparse arrays (e.g. [1, , 3]) are skipped entirely by map, filter, forEach, etc. Callback is never executed for unassigned slots.

⚠️ includes() vs indexOf() for NaN [NaN].includes(NaN) returns true (using SameValueZero algorithm), whereas [NaN].indexOf(NaN) returns -1 because NaN !== NaN!

Array Method Quick Comparison Table

Compare return types, mutability, short-circuiting behaviors, and best-practice use cases at a glance.

Method	Returns	Mutates?	Short-circuits?	Typical Use Case

Interactive Method Reference

Detailed syntax, return semantics, signatures, gotchas, and one-click runnable examples.