03-bugfix-numbers

Exercise 03 - Bug Hunt: Numbers & Money

The scenario

Five utility functions for the park's financial and sensor systems have number-handling bugs. IEEE 754 floating point makes 0.1 + 0.2 !== 0.3, parseInt silently ignores trailing garbage, the global isNaN lies about strings, division by zero produces Infinity instead of an error, and .toFixed() returns a string that looks like a number but concatenates instead of adding.

These bugs are responsible for more production incidents than most people realise. You'll fix each one using the correct numeric tools.

How it works

1. Run the tests — they will fail.
2. Read each failure to understand the expected behaviour.
3. Fix the bug in starter/numbers.js. Each function has exactly one bug.

The bugs you are hunting

Function	What it does	The bug
totalPriceCents(prices)	Sum an array of dollar prices and return the total in cents	Sums dollars first then converts — accumulates IEEE float errors. Should convert each price to cents first
parseAge(input)	Parse a string to an integer age, return null for invalid input	parseInt("12abc") returns 12 — should reject strings that aren't purely numeric
isStrictlyNaN(val)	Return true only for NaN	Uses global isNaN() which coerces — isNaN(undefined) and isNaN("hello") wrongly return true
safeDivide(a, b)	Divide a by b, return null if b is zero	Returns Infinity or NaN on division by zero instead of null
roundTo2(n)	Round to 2 decimal places and return a number	.toFixed(2) returns a string — arithmetic on the result silently concatenates

Getting started

Open starter/numbers.js. Run starter/index.js to see the broken output:

node starter/index.js

Verify

cd starter && pnpm install && pnpm test

Hints

• For money: work in cents (integers) to avoid float precision issues. Math.round(price * 100) converts each dollar amount to cents safely.
• Number(input) is stricter than parseInt — Number("12abc") gives NaN.
• Number.isNaN(val) is the non-coercing version of isNaN().
• Check b === 0 before dividing.
• Wrap .toFixed(2) in Number(...) or use parseFloat(...) to get back to a number.

Reference solution

solution/numbers.js