JavaScript Challenges

Cached loader runs 3 times instead of once

countLoads(n) calls a cached loader n times at once and returns how many times the slow load really ran. It should always be 1. Example: await countLoads(3) → 1.

Async race condition

Doubling after a delay returns undefined

doubleLater(value) waits a moment, then gives back value × 2. Example: await doubleLater(4) → 8.

Async race condition

getName gives back undefined

getName(id) loads a user and gives back their name. Example: await getName(1) → "user1".

Async race condition

Loading 3 items returns only one

loadAll(ids) loads every id and returns all the results in order. Example: await loadAll([1, 2, 3]) → [10, 20, 30].

Async race condition

Adding 4 votes at once keeps only the last

tallyVotes(votes) adds up every vote. The votes are counted at the same time with Promise.all. Example: await tallyVotes([1, 2, 3, 4]) → 10.

Async race condition

try/catch doesn't catch the divide-by-zero error

safeDivide(a, b) returns a / b, or -1 if the division fails. Example: await safeDivide(10, 2) → 5, await safeDivide(10, 0) → -1.

Async race condition

Async sum gives "[object Promise]3"

sumAsync(nums) adds up values that are fetched with an async function. Example: await sumAsync([1, 2, 3]) → 6.

Async race condition

Names come back in finishing order, not input order

labelAll(items) returns the names in the same order as the input, however long each one takes. Example: await labelAll([{ name: "a", ms: 20 }, { name: "b", ms: 1 }]) → ["a", "b"].

Async race condition

Cart total ignores how many of each item

cartTotal(items) adds up price × quantity for every item. This project has two files: cart.js and lineItem.js. Each item looks like { price: 10, quantity: 2 }. Example: [{ price: 10, quantity: 2 }, { price: 5, quantity: 1 }] → 25.

Logic error

Grouping words keeps only one per group

groupByLength(words) groups words by their length. Example: groupByLength(["ab", "cd", "x"]) → { 1: ["x"], 2: ["ab", "cd"] }.

Logic error

Capitalize turns "debug" into "Ddebug"

capitalize(word) makes the first letter a capital. Example: capitalize("debug") → "Debug".

Off-by-one

Flattening stops after one level

flattenDeep(arr) removes all nesting, however deep. Example: flattenDeep([1, [2, [3, [4]]]]) → [1, 2, 3, 4].

Other

0.1 + 0.2 gives 0.30000000000000004

addPrices(a, b) adds two prices and returns a result correct to the cent. Example: addPrices(0.1, 0.2) → 0.3.

Other

Total shows "[object Promise]" instead of 6

sumDelayed(nums) adds up numbers that each arrive after a short delay. Example: await sumDelayed([1, 2, 3]) → 6.

Async race condition

Doubling with forEach returns an empty list

doubleAll(nums) doubles every number with fetchDouble, which takes a moment, and returns the results. Example: await doubleAll([1, 2, 3]) → [2, 4, 6].

Async race condition

Word lengths come back as promises, not numbers

wordLengths(words) returns the length of each word. Example: await wordLengths(["abc", "defg"]) → [3, 4].

Async race condition

Every callback reports index 3

collectIndexes(n) starts one delayed callback for each index and returns the indexes they report. Example: collectIndexes(3) → [0, 1, 2].

Scope error

Vote total shows "034" instead of 7

countVotes(entries) adds up the votes of every entry. The votes arrive as text, like "3". Example: votes "3" and "4" → 7.

Type error

Member discount crashes with ReferenceError

finalPrice(price, isMember) gives members 10% off and returns the price to pay. Example: finalPrice(100, true) → 90, finalPrice(100, false) → 100.

Scope error

Sorting puts 9 after 100

sortNumbers(nums) returns the numbers from smallest to largest. Example: sortNumbers([10, 9, 100, 1]) → [1, 9, 10, 100].

Type error

Removing duplicates never finishes

dedupe(arr) returns the array without duplicates, keeping the first copy of each value. Example: dedupe([1, 2, 2, 3]) → [1, 2, 3].

Infinite loop

Reversing a word never finishes

reverseString(text) returns the text backwards. Example: reverseString("abc") → "cba".

Infinite loop

Free shipping not given at exactly 50

qualifiesForFreeShipping(total) returns true when the order is 50 or more. Example: qualifiesForFreeShipping(50) → true, qualifiesForFreeShipping(49.99) → false.

Logic error