Eye Training · Values, Equality and Numbers

JavaScript converts values quietly. Every bug here is a value that was quietly converted into something else.

Part 2 · Surface 19. Bug counts are hidden and some snippets are clean. See Start Here for how to use these drills.

J · The basics

M · Three years in, these should jump out

S · Where seniors stand out

What your eye checks

Whenever a value is compared, defaulted, converted or used in arithmetic, ask:

1. Which falsy values are valid here? 0, '' and false are often real values. || and if (!x) throw them away; ?? and == null don't.
2. What type is it really? Form fields, query strings, environment variables and headers are strings. + concatenates strings.
3. What does the conversion do with odd input? Number('') is 0, Number('12px') is NaN, parseInt('12px') is 12, global isNaN converts first.
4. Is it money? Integers in the smallest unit, or a decimal library. Never floats; never toFixed until display.
5. Is NaN checked with Number.isNaN? And is null excluded before typeof x === 'object'?
6. Are numbers or versions compared as strings?
7. Can it exceed the safe range? 64-bit ids beyond 2⁵³ − 1; bitwise operators truncate to 32 bits.
8. What does the empty case return? Math.max(), Math.min(), averages of nothing (0 / 0 is NaN).

Tells

Go deeper

• L1–L2 · Logic and TypeScript: the same slips in bigger files, and where TypeScript does and doesn't catch them
• Previous surface: Shared state and races · Next: Objects and arrays