What is the default sort()?

String comparison by UTF-16 code units, in place.

What is the toSorted()?

Copying version of sort(); original unchanged.

What is the slice(start, end)?

end excluded; original unchanged.

What is the reduce() without initial value?

Accumulator starts as array[0]; empty array throws TypeError.

Variables, Types, and Collections23% of the exam

Array methods for data manipulation

Which array methods mutate and which return copies, what each one returns (map, filter, reduce, find, findIndex, some, every, includes, flat, forEach), the default string sort and comparators, shallow copies with spread, array destructuring, and Set and Map as collections.

Official exam objective

Given a list of data, demonstrate data manipulation with arrays.

~6 min read · 14 practice questions · updated Oct 16, 2026

What the exam expects

Tell mutating methods (push, pop, shift, unshift, splice, sort, reverse, fill) from copying ones (concat, slice, map, filter, flat, spread) and predict what each returns.
Pick the right query method for a requirement: find, findIndex, filter, some, every, includes, indexOf.
Sort numbers and objects with a comparator, and total values with reduce and an initial value.
Copy arrays safely (shallow copies, fill with objects, Array.from) and unpack them with destructuring.
Remove duplicates and key data by any value with Set and Map.
Key terms: mutating method (changes the array it is called on); shallow copy (a new array whose object elements are still the same references); callback (the function an iterative method calls with element, index and array); comparator (a sort function returning negative, zero or positive); sparse array (an array with empty slots).

Key facts

Fact	Detail	Source
Default `sort()`	String comparison by UTF-16 code units, in place	Array.prototype.sort() (opens in a new tab)
`toSorted()`	Copying version of `sort()`; original unchanged	Array.prototype.toSorted() (opens in a new tab)
`sort()` return	The same array reference	Array.prototype.sort() (opens in a new tab)
`push()` return	New length	Array.prototype.push() (opens in a new tab)
`splice()` return	Removed elements; mutates	Array.prototype.splice() (opens in a new tab)
`slice(start, end)`	`end` excluded; original unchanged	Array.prototype.slice() (opens in a new tab)
`forEach()` return	`undefined`	Array.prototype.forEach() (opens in a new tab)
`reduce()` without initial value	Accumulator starts as `array[0]`; empty array throws `TypeError`	Array.prototype.reduce() (opens in a new tab)
`[].every(fn)`	`true`	Array.prototype.every() (opens in a new tab)
`indexOf(NaN)` / `includes(NaN)`	-1 / `true`	Array.prototype.includes() (opens in a new tab)
`flat()` depth	Defaults to 1	Array.prototype.flat() (opens in a new tab)
Spread, `slice`, `concat`, `Array.from`	Shallow copies	Array (opens in a new tab)
`fill(obj)`	Every slot references the same object	Array.prototype.fill() (opens in a new tab)
`Map` keys	Any value; objects matched by identity	Map (opens in a new tab)

Default sort()

Detail: String comparison by UTF-16 code units, in place
Source: Array.prototype.sort() (opens in a new tab)

toSorted()

Detail: Copying version of sort(); original unchanged
Source: Array.prototype.toSorted() (opens in a new tab)

sort() return

Detail: The same array reference
Source: Array.prototype.sort() (opens in a new tab)

push() return

Detail: New length
Source: Array.prototype.push() (opens in a new tab)

splice() return

Detail: Removed elements; mutates
Source: Array.prototype.splice() (opens in a new tab)

slice(start, end)

Detail: end excluded; original unchanged
Source: Array.prototype.slice() (opens in a new tab)

forEach() return

Detail: undefined
Source: Array.prototype.forEach() (opens in a new tab)

reduce() without initial value

Detail: Accumulator starts as array[0]; empty array throws TypeError
Source: Array.prototype.reduce() (opens in a new tab)

[].every(fn)

Detail: true
Source: Array.prototype.every() (opens in a new tab)

indexOf(NaN) / includes(NaN)

Detail: -1 / true
Source: Array.prototype.includes() (opens in a new tab)

flat() depth

Detail: Defaults to 1
Source: Array.prototype.flat() (opens in a new tab)

Spread, slice, concat, Array.from

Detail: Shallow copies
Source: Array (opens in a new tab)

fill(obj)

Detail: Every slot references the same object
Source: Array.prototype.fill() (opens in a new tab)

Map keys

Detail: Any value; objects matched by identity
Source: Map (opens in a new tab)

Sources for this section (13)

Check yourself

Why does `new Array(3).fill([])` make all three slots change together?
When should you use `find()` instead of `filter()`?
Why does `readings.indexOf(NaN)` return -1 while `includes(NaN)` returns `true`?
What does `[].every((x) => x > 0)` return?
Which expression returns unique tags as an array in original order?

The questions are free to read. Answers with their sources and self-grading are included with the pack.

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What the exam expects

Key facts

Check yourself

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