Find the bug · Easy

Fix the Number Parser (map + parseInt)

Easy Find the bug

parseAll(strings) converts an array of decimal integer strings (such as query-string values) into numbers.

The one-liner below looks idiomatic. It works for one value and fails for lists. Find the bug and fix it.

Example 1

Input: strings = ["7"]
Output: [7]

Example 2

Input: strings = ["10", "10", "10"]
Output: [10, 10, 10]
Explanation: The buggy version returns [10, NaN, 2].

The buggy code

function parseAll(strings) {
  return strings.map(parseInt);
}

function parseAll(strings: string[]): number[] {
  return strings.map(parseInt);
}

Hints

Hint 1

What arguments does map pass to its callback?

Hint 2

What is the second parameter of parseInt?

Find and fix the bug

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Solution and explanation

Show the bug and the fix

Time O(n) · Space O(n)

The bug: map calls its callback with (element, index, array), and parseInt(string, radix) takes the index as the radix. parseInt("10", 1) is NaN (radix 1 is invalid) and parseInt("10", 2) is 2. Only index 0 works, because radix 0 means "guess".

The fix: pass only the element and an explicit radix: strings.map((s) => parseInt(s, 10)), or use Number (strings.map(Number)), which takes one argument.

Fixed version

function parseAll(strings) {
  return strings.map((s) => parseInt(s, 10));
}

function parseAll(strings: string[]): number[] {
  return strings.map((s) => parseInt(s, 10));
}

Common mistakes

map(Number) accepts "" as 0 and "1e3" as 1000: say which behaviour you want.

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