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Code Completion 12 Challenges

Complete the JavaScript keyed collections code

Practice Map association, Set membership and algebra, and WeakMap object-keyed metadata.

Intermediate ~8 min read

Challenge 1

Associate a user ID with a record.

javascript Your answer 
const users = new Map();
users.set(42, { name: "Ada" });

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Challenge 2

Read the record for ID 42.

javascript Your answer 
const record = users.get(42);

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Challenge 3

Test whether ID 42 exists.

javascript Your answer 
const known = users.has(42);

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Challenge 4

Track a completed user ID.

javascript Your answer 
const completed = new Set();
completed.add(42);

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Challenge 5

Test whether a user ID is completed.

javascript Your answer 
const done = completed.has(42);

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Challenge 6

Keep only shared capabilities.

javascript Your answer 
const shared = first.intersection(second);

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Challenge 7

Combine capability sets without duplicates.

javascript Your answer 
const all = first.union(second);

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Challenge 8

Associate metadata with an object identity without needing enumeration.

javascript Your answer 
const cache = new WeakMap();
cache.set(element, metadata);

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Challenge 9

Return the existing value or insert a default value.

javascript Your answer 
const value = settings.getOrInsert("theme", "light");

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Challenge 10

Lazily create a missing Map value.

javascript Your answer 
const value = settings.getOrInsertComputed("theme", key => makeTheme(key));

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Challenge 11

Associate metadata with an object only when that object is not already cached.

javascript Your answer 
const metadata = cache.getOrInsert(element, createMetadata(element));

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Challenge 12

Lazily create metadata for an uncached object.

javascript Your answer 
const metadata = cache.getOrInsertComputed(element, key => createMetadata(key));

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