Challenge Library
Coding Challenges
5,893 curated questions across every arena — browse by topic, then jump into a live 3-minute battle to put them to the test.
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345 challenges matching your filters
- Code AnalysisHard
setTimeout in setTimeout
javascripthard - Code AnalysisHard
Loop closure var
javascripthard - Code AnalysisHard
Loop closure let
javascripthard - Code AnalysisHard
module.exports overwrite
javascripthard - Code AnalysisHard
ESM top-level await
javascripthard - Code AnalysisHard
Object.keys order
javascripthard - Code AnalysisHard
Promise executor sync
javascripthard - Code AnalysisHard
Array coercion
javascripthard - Code AnalysisHard
IIFE Semicolon Trap
code-analysisJavaScriptlanguage - Code AnalysisHard
Proxy Trap
code-analysisJavaScriptlanguage - Code AnalysisHard
Sparse Array
code-analysisJavaScriptlanguage - Code AnalysisHard
String Coercion
code-analysisJavaScriptlanguage - Code AnalysisHard
Type Coercion Madness
code-analysisJavaScriptlanguage - Code AnalysisHard
Metaclass Instantiation
pythonhard - Code AnalysisHard
__new__ singleton
pythonhard - Code AnalysisHard
descriptor access
pythonhard - Code AnalysisHard
init_subclass
pythonhard - Code AnalysisHard
singledispatch
pythonhard - Code AnalysisHard
Trace: Late Binding Closure
pythonanalysishard - Code AnalysisHard
Lock acquire
pythonhard - Code AnalysisHard
raise from
pythonhard - Code AnalysisHard
weakref behavior
pythonhard - Code AnalysisHard
sys.getsizeof — Overhead
pythonhard - Code AnalysisHard
__slots__ inheritance
pythonhard - Code AnalysisHard
typing Union
pythonhard - Code AnalysisHard
Literal type
pythonhard - Code AnalysisHard
Class Variable Shadow
code-analysisPythonlanguage - Code AnalysisHard
Decorator Order
code-analysisPythonlanguage - Code AnalysisHard
is vs == (int cache)
code-analysisPythonlanguage - Code AnalysisHard
Multiple Inheritance MRO
code-analysisPythonlanguage - Code AnalysisHard
Mutable Default Arg
code-analysisPythonlanguage - Code AnalysisHard
String Interning
code-analysisPythonlanguage - Code AnalysisHard
Walrus in Comprehension
code-analysisPythonlanguage - Code AnalysisHard
N+1 query in an ORM loop
system-designdatabasesn-plus-one - Code AnalysisHard
Cache stampede on expiry
system-designcachingstampede - Code AnalysisHard
Read-modify-write lost update
system-designconcurrencydatabases - Code AnalysisHard
Hot shard from a skewed key
system-designshardinghot-partition - Code AnalysisHard
Sync replication availability trap
system-designreplicationcap-theorem - Code AnalysisHard
Fan-out-on-write celebrity
system-designnews-feedfan-out - Code AnalysisHard
Split-brain from naive failover
system-designconsensusavailability - Code AnalysisHard
Outbox without proper relay
system-designoutboxdistributed-transactions - Code AnalysisHard
Thundering herd with negative result
system-designcachingcache-penetration - Code AnalysisHard
Unbounded cardinality metric
system-designobservabilitymetrics - Code AnalysisHard
CAS retry livelock
system-designconcurrencydistributed-counter - Code AnalysisHard
Retry without idempotency on a non-idempotent path
system-designidempotencyretry - Code AnalysisHard
At-least-once with side effect before ack
system-designmessagingidempotency - Code AnalysisHard
Fencing missing on lease-based lock
system-designconsensusleader-election - Code AnalysisHard
Backpressure via unbounded broker retention
system-designmessagingbackpressure - Code AnalysisHard
Quorum read after failed write
system-designquorumconsistency - Code AnalysisHard
Idempotency key stored after the effect
system-designidempotencyconcurrency - Code AnalysisHard
Fan-out read amplification
system-designnews-feedfan-out - Code AnalysisHard
Offset commit before processing
system-designmessagingkafka - Code AnalysisHard
Unbounded retry on non-transient error
system-designresilienceretry - Code AnalysisHard
CAS without version on concurrent edit
system-designconcurrencydatabases - Code AnalysisHard
Bloom filter with no eviction
system-designcachingbloom-filter - Code AnalysisHard
Commit before process loses messages
kafkaoffset-commitmessage-loss - Code AnalysisHard
Lost update from read-modify-write
transactionslost-updateconcurrency - Code AnalysisHard
Phantom read with range predicate
phantom-readisolation-levelsmvcc - Code AnalysisHard
LSM write amplification analyzer
lsmcompactionrocksdb - Code AnalysisHard
Two-phase commit blocking analysis
2pcdistributed-transactionsblocking
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