Keentune

JavaScript curriculum

24 chapters
·
239 concepts
·
free
Everything the adaptive question bank can teach and test in JavaScript, from foundations through advanced practice. Work through it in order, or start practising and let the questions find your level.
New here? Read the JavaScript guide
A free 17-minute primer — the mental model, the mistakes beginners make, and what to practise first.
A. Grammar, declarations and types
•
var is function-scoped; let and const are block-scoped
•
a let binding exists from block entry but throws until its declaration runs
•
const forbids reassignment, not mutation of the referenced object
•
a function declaration hoists with its body; var hoists initialized to undefined
•
absent by default versus deliberately empty, and why typeof null is "object"
•
seven primitive types, everything else a reference; primitives are boxed on property access
•
the fixed list typeof can return, including "function" and the null bug
•
a newline after return ends the statement and returns undefined
•
what "use strict" changes: silent write failures throw, this is no longer boxed
•
sloppy-mode assignment to an undeclared name creates a global; globalThis names it portably
•
positional binding, skipped holes, a rest element collecting the remainder, and [a, b] = [b, a] as the swap idiom. Source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring
•
matching by key rather than position, renaming with { key: newName }, and rest properties. Source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring
•
a default fires only when the value is undefined, so an explicit null is kept and a present value is never overridden. Source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring
B. Operators, coercion and equality
•
== runs the abstract-equality coercion steps; === compares type and value directly
•
null == undefined is true and null == 0 is not; the pair is equal only to each other
•
Object.is distinguishes +0 from -0 and reports NaN as equal to itself
•
==, ===, SameValue and SameValueZero, and which built-in reaches for which
•
+ concatenates if either operand becomes a string; every other arithmetic operator coerces to number
•
Symbol.toPrimitive first, then valueOf/toString in an order set by the hint
•
exactly false, 0, -0, 0n, "", null, undefined, NaN; "0" and [] are truthy
•
&& and || evaluate to one of their operands, not to a boolean
•
?? falls through only on null or undefined, so 0 and "" survive
•
&&=, ||= and ??= short-circuit the assignment itself, skipping the setter
•
?. abandons the rest of the chain and yields undefined rather than throwing
•
** groups right to left and rejects an unparenthesized unary minus on its left
•
a ?? b || c will not parse; the grammar demands parentheses
•
operands are truncated to int32, so >>> and >> disagree on negatives and | 0 floors
•
< and > coerce each operand to a primitive and then a number, so 3 > 2 > 1 evaluates true > 1 and is false
•
the comma operator evaluates every expression left to right and yields the LAST one, which is why (1, 2, 3) is 3
C. Control flow, errors and cleanup
•
anything can be thrown, which is why a caught value has no guaranteed shape
•
TypeError, RangeError, ReferenceError and SyntaxError each signal a different failure class
•
extends Error, setting name, and keeping the stack and message intact
•
the { cause } option preserves the original failure instead of flattening it (ES2022)
•
including on return and on rethrow, and a return inside finally discards the pending result
•
catch {} when the error value is genuinely unused
•
Error.isError identifies errors across realms where instanceof fails (ES2026)
•
a case without break continues into the next, and switch matches with ===
•
using disposes at scope exit via Symbol.dispose, LIFO, even when the block throws (stage 4)
D. Loops and iteration statements
•
enumerable string-keyed properties, inherited ones included
•
requires an iterable and never sees prototype properties
•
indices arrive as strings, which is why for...in is the wrong array loop
•
let in a for head creates a fresh binding each pass; var creates one for all of them
•
a name attached to a loop, so break and continue can target it instead of the innermost one
•
forEach ignores return and has no early exit; some/every/for...of do
•
consumes an async iterable and is sequential by construction
•
the condition is tested after the body, so the body always executes at least once
E. Functions and parameters
•
hoisting, the named-function-expression scope, and inferred names
•
evaluated at call time, left to right, and only when the argument is undefined
•
...args is a real array; arguments is array-like and absent in arrow functions
•
spreading any iterable into an argument list, and the array and object forms
•
object patterns, inner defaults, and why a missing argument throws without = {}
•
fn.length counts parameters before the first default or rest parameter
•
no own this, arguments, super or new.target, and not constructible
•
an immediately invoked expression as the pre-module isolation trick
•
engines do not implement tail calls, so deep recursion is a RangeError
•
the reference is copied, so reassigning a parameter never affects the caller
F. Binding, this and closures
•
this is resolved when the function is called, not where it was written
•
passing obj.m as a callback strips the receiver
•
an explicit receiver, a spread argument list, and a permanently bound copy
•
an arrow closes over the enclosing this and cannot be rebound
•
new allocates, links the prototype and returns the object unless the body returns another object
•
an undecorated call gets undefined in strict mode and the global object otherwise
•
a closure sees the variable's later value, not a snapshot taken at creation
•
var leaves every callback in a loop sharing one variable
•
data no caller can reach, paid for with per-instance memory
•
fixing some arguments now and taking the rest later
G. Objects and properties
•
any other key is coerced to a string, so obj[1] and obj["1"] are one property
•
{ [expr]: v } builds the key at runtime; { x } is { x: x }
•
value, writable, enumerable, configurable, and how defineProperty defaults them to false
•
accessor properties run code on read or write, and are invisible at the call site
•
Object.hasOwn ignores the prototype chain; the in operator walks it
•
Object.freeze protects one level, so nested objects remain mutable
•
which of adding, deleting and writing each one still permits
•
both copy only own enumerable properties, and copy references not values
•
Object.assign triggers an inherited setter; spread creates an own data property
•
integer-like keys ascend first, then string keys in insertion order, then symbols
•
round-tripping an object through an array of pairs to filter or transform it
•
removes the property entirely, unlike setting it to undefined, and leaves an array hole
•
Object.assign applies its sources left to right, so a later source overwrites an earlier one key by key
H. Prototypes and inheritance
•
a missing property walks [[Prototype]] links until it reaches null
•
an instance's link versus the object a constructor hands to its instances
•
constructing an object with a chosen prototype, including no prototype at all
•
writing an inherited property creates an own property instead of updating the parent
•
if the prototype defines a setter, the write runs it and no own property appears
•
it tests the chain, not the constructor, and breaks across realms
•
inherited and writable, so it lies after you replace prototype wholesale
•
a safe dictionary with no inherited methods, and what stops working on it
•
one function object on the prototype serves every instance
I. Classes
•
methods land on prototype, but the body is always strict and the binding is not usable before evaluation
•
a derived constructor must call super() before it may touch this
•
instance fields are created per instance, in declaration order, before the constructor body
•
a class field holding an arrow is per-instance and survives detachment
•
#x is a hard-private slot enforced by the engine, and #x in obj is the brand check
•
statics live on the constructor; a static block runs once when the class is defined
•
get/set install accessor properties on the prototype, not on the instance
•
subclassing Array or Error, and how Symbol.species decides what methods return
•
distinguishing construction from a plain call inside the same function
•
super.m() resolves against the home object while this stays the current instance
•
a function returning class extends Base to compose behavior without multiple inheritance
J. Numbers and arithmetic
•
every Number is a 64-bit binary float, which is why 0.1 + 0.2 !== 0.3
•
beyond Number.MAX_SAFE_INTEGER integer arithmetic silently returns the wrong answer
•
the only value failing x === x; test it with Number.isNaN
•
global isNaN("foo") is true because it converts first; Number.isNaN does not
•
prefix parsing with an explicit radix versus whole-string conversion that rejects trailing junk
•
and it rounds the stored binary value, so (1.005).toFixed(2) is not "1.01"
•
where -0 is observable and where === hides it
•
-7 % 3 is -1; a true modulo needs ((n % m) + m) % m
•
Math.round breaks ties toward +Infinity, which differs from trunc, floor and ceil on negatives
•
arbitrary-precision integers throw when combined with a Number in arithmetic
•
toString(radix) renders a number in another base, using lowercase digits above 9
•
Math.max() with no arguments returns -Infinity (and min() returns Infinity) because each is the identity for its operation; plus sign, hypot, cbrt, and sqrt of a negative giving NaN. Source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/max
•
1/0 and -1/0 are Infinity and -Infinity while 0/0 is NaN; all three are ordinary Number values rather than errors
•
Number("") and Number(null) are 0, but Number(undefined) is NaN
K. Strings and text
•
no method mutates in place; every operation returns a new string
•
length counts code units, so an emoji or an astral character counts as two
•
spread and for...of walk code points; bracket indexing splits surrogate pairs
•
NFC versus NFD, and why two identical-looking strings can be unequal
•
slice accepts negative indices; substring clamps and silently swaps its arguments
•
a string pattern replaces only the first match unless the regex carries /g
•
$1, $& and $$ in the string form, and the argument list of the function form
•
interpolation, embedded expressions, and literal newlines
•
a function receiving the literal chunks and the interpolated values separately, plus .raw
•
< compares code units; localeCompare applies collation rules
•
the argument is the final total length, and a longer string is returned unchanged
•
split cuts a string into an array on a separator; join collapses one back, and the empty separator splits by code unit
•
trim strips both ends while trimStart and trimEnd take only one, and none of them touch interior whitespace
•
at(-1) reads back from the end, which bracket indexing cannot do
•
String(null) and ` ${null} both give "null", while null.toString()` throws
L. Arrays and indexed collections
•
assigning a smaller length truncates; writing past the end extends with holes
•
iteration methods skip holes while for loops and spread see undefined
•
[10, 9].sort() stays [10, 9] because elements are compared as strings
•
it sorts in place, returns the same array, and preserves the order of ties
•
splice/reverse/sort versus toSpliced/toReversed/toSorted/with (ES2023)
•
one output per input, a filtered subset, and a fold over an accumulator
•
without one the first element seeds the accumulator, and an empty array throws
•
the first match, every match, or a position
•
indexOf uses strict equality and cannot find NaN; includes uses SameValueZero
•
depth-limited flattening, and mapping then flattening one level in a single pass
•
building from an iterable or array-like with an optional mapper, versus from a literal argument list
•
["1","2","3"].map(parseInt) passes the index as the radix
•
the array form of at, mirroring the string one, for reading the last element without length - 1
M. Keyed collections and weak references
•
a Map takes any value as a key, guarantees insertion order, and has no prototype keys to collide with
•
two structurally identical objects are two distinct keys
•
adding an existing member is a no-op; 0 and -0 collapse and so do two NaNs
•
union, intersection, difference, symmetricDifference and the subset predicates (ES2025)
•
Map.size is a property; counting keys of a plain object allocates an array first
•
the entry disappears when the key becomes unreachable, and the contents are not enumerable
•
deliberately non-deterministic, so never a cleanup guarantee
•
Map.getOrInsert and getOrInsertComputed replace the has-get-set dance (ES2026)
N. Iterators and generators
•
an object is iterable when Symbol.iterator returns an iterator; that is all for...of requires
•
next() returns { value, done }, and a spent iterator stays done
•
function* and yield suspend and resume with local state intact
•
yield* forwards to another iterable and evaluates to that generator's return value
•
the argument passed to next() becomes the value of the paused yield
•
break invokes the iterator's return(), which is where a generator's finally runs
•
producing values on demand instead of materializing a whole collection
•
map, filter, take, drop and toArray chained lazily on any iterator (ES2025)
•
Symbol.asyncIterator yields promises, and for await unwraps each one
O. Promises and async/await
•
pending, fulfilled or rejected; a promise settles once and never changes again
•
returning a promise from a handler flattens it into the chain
•
the new Promise body executes immediately; only the reactions are deferred
•
a catch placed before a then cannot handle that then's rejection
•
a promise created inside a handler but not returned is not awaited by the chain
•
reject on the first failure versus collect every outcome
•
first to settle, versus first to fulfill with an AggregateError when all reject
•
always, and a synchronous throw inside it becomes a rejection
•
start every request first and await the array to run them concurrently
•
attaching a handler in a later task is too late for the runtime's warning
•
Promise.try wraps a possibly-synchronous throw; withResolvers hoists resolve and reject out (ES2024/ES2025)
•
a promise cannot be cancelled; you signal the producer, typically with an AbortSignal
•
await adopts any object with a callable then, including one that never settles
P. The concurrency model and the event loop
•
a task finishes before any other task starts, so nothing interleaves mid-function
•
the microtask queue drains completely between two tasks
•
which is why a resolved then runs before a zero-delay timer
•
it queues a task and is clamped, so ordering with promises is fixed, not racy
•
one long synchronous computation delays every timer, promise and input handler
•
the code after an await is scheduled, so it never runs in the same synchronous pass
•
a microtask that queues another microtask can prevent the task from ever ending
•
debounce fires once after calls STOP arriving; throttle fires at most once per interval while they keep arriving
Q. Modules
•
renaming with as, and why a default import's local name is arbitrary
•
bindings exist before the body runs, and the specifier must be a literal
•
an imported binding reflects a later reassignment in the exporting module
•
assigning to an imported binding is a TypeError, even for a mutable object's variable
•
top-level declarations stay local, and module code is always strict
•
the registry caches the instance, so import side effects run exactly once per specifier
•
import() returns a promise, works in any scope, and is how code splitting happens
•
export * from versus explicit re-export, and how name collisions are resolved
•
a cycle can observe a binding before its initializer has run
•
with { type: "json" } declares the expected module type (ES2025)
R. Dates and times
•
a Date is a single instant stored as milliseconds since the 1970 UTC epoch
•
while days of the month are not
•
"2026-01-01" parses as UTC but "2026-01-01T00:00" parses as local time
•
setters modify the instance, so a shared Date is an aliasing bug waiting to happen
•
setting day 32 rolls forward into the next month rather than throwing
•
getHours reads the host time zone; getUTCHours does not, and DST breaks naive day arithmetic
•
a failed parse yields a Date whose time value is NaN, which compares false against everything
•
why Temporal exists: immutable values, explicit time zones and calendar-aware arithmetic (stage 4)
S. Regular expressions
•
a RegExp built from a string needs every backslash doubled
•
a /g regex carries lastIndex, so reusing it across calls skips matches
•
a boolean, a single match with its groups, or a lazy iterator over every match
•
with /g, match returns bare strings and the capture groups are gone
•
(?<name>…) and reading results from groups instead of by index
•
.* versus .*?, and where catastrophic backtracking comes from
•
/u matches whole code points and enables \p{…}; /v adds set operations
•
RegExp.escape neutralizes untrusted text before it becomes a pattern (ES2025)
•
(?i:…) and (?-i:…) apply a flag to one group only, leaving the rest of the pattern under the pattern-level flags
T. JSON and serialization
•
omitted from objects, but serialized as null inside arrays
•
an object can define its own serialized form, which is exactly how Date becomes a string
•
a key whitelist versus a per-key transform invoked top-down
•
the parse hook sees innermost values first, which is how types are restored
•
JSON.stringify raises a TypeError rather than truncating
•
Map, Set, BigInt, undefined and prototypes do not survive the trip
U. Symbols and metaprogramming
•
no two symbols are equal, and symbol keys are skipped by Object.keys and JSON.stringify
•
Symbol.for returns a shared registry symbol; Symbol() never repeats
•
Symbol.toStringTag changes what Object.prototype.toString.call reports about a value
•
get, set, has and deleteProperty intercept the operation itself, not a copied value
•
a trap that contradicts a non-configurable, non-writable property throws a TypeError
•
Reflect supplies the correct default behavior for each trap, with a boolean return
•
forwarding receiver through Reflect.get is what keeps an inherited getter's this correct
•
handing out access you can withdraw later
•
new Function compiles in global scope, both cost optimization, and both are injection surface
V. Typed arrays and binary data
•
the buffer is raw bytes; all reads and writes go through a typed array or DataView
•
two views over one buffer observe each other's writes immediately
•
Uint8Array wraps modulo 256 while Uint8ClampedArray saturates at the bounds
•
DataView takes an explicit byte-order argument that typed arrays do not
•
no push, and an out-of-range index write is silently discarded
•
Uint8Array.fromBase64 and toHex replace the old string round trip (ES2026)
•
Float16Array trades range and precision for size, so not every value round-trips (ES2025)
W. Internationalization
•
grouping separators, currency and unit display differ by locale
•
rendering one instant in a named IANA zone without mutating the date
•
locale-correct ordering, with options for case and accent sensitivity
•
"3 days ago" and correct plural categories without hand-written rules
•
the requested locale list, the fallback chain, and reading resolvedOptions
X. Language evolution and version gates
•
stage 0 through 4, and why only stage 4 may be taught as the language
•
backward compatibility is absolute, which is why the historical mistakes are permanent
•
test for the method or syntax, never for a runtime version number
•
syntax is compiled down; a missing built-in has to be added at runtime
•
with, substr and escape are specified for compatibility and still must not be used
•
setTimeout, console and module loading come from the host, not from ECMAScript
Keentune is not affiliated with or endorsed by the organizations whose documentation informs these maps.
All about JavaScript practice
Also on your phone
App StoreGoogle Play
All exam, test, and product names and trademarks are the property of their respective owners and are used here for identification and reference only. Keentune is independent study practice — not affiliated with, authorized, or endorsed by any of these organizations.
© 2026 SportaApp LLC