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Use ?? For Defaults, Not ||

This JavaScript default-value bug is probably in your codebase right now. config.port || 3000 looks perfectly reasonable — and it silently rewrites a port of 0 to 3000, an empty prefix to your fallback string, and an explicit false to true. The fix is two characters: ??.

The bug: || tests truthiness, not presence

x || y returns y whenever x is falsy. JavaScript has six falsy values: false, 0, "" (empty string), null, undefined, and NaN. Four of those are perfectly valid configuration values:

const port    = config.port    || 3000;   // 0      -> 3000  (bug)
const prefix  = config.prefix  || "app";  // ""     -> "app" (bug)
const retries = config.retries || 5;      // 0      -> 5     (bug)
const debug   = config.debug   || true;   // false  -> true  (bug!)

The user set those values. || throws them away because it can't tell "absent" from "falsy". port: 0 is a real request — "give me a random free port" in Node's server.listen(0) — and debug: false is the single most common way to turn something off.

The fix: ?? tests nullish, not falsy

The nullish coalescing operator (??, ES2020) falls back only when the left side is null or undefined:

const port    = config.port    ?? 3000;   // 0      stays 0
const prefix  = config.prefix  ?? "app";  // ""     stays ""
const retries = config.retries ?? 5;      // 0      stays 0
const debug   = config.debug   ?? true;   // false  stays false

Absent key? config.port is undefined, you get 3000. Explicit null? You get 3000. Anything else — including every falsy value the user meant — survives.

Rules of thumb

Keep || only when you genuinely want to treat all falsy values as missing — e.g. normalizing user text where "" should become a placeholder. For config, options objects, and API responses, default with ??.

Power-user corner

• ??= for options objects (ES2021). options.timeout ??= 100 assigns only when the property is null/undefined — a one-liner replacement for the if (options.timeout == null) dance, and it short-circuits (the right side isn't even evaluated otherwise).
• Pair with optional chaining. customer?.address?.city ?? "Unknown" never throws on a missing branch and only defaults on true absence — the idiomatic deep-read-with-default.
• Both short-circuit. a ?? expensiveDefault() never calls the function when a is present — safe for lazy/costly fallbacks.
• Parenthesize when mixing. a || b ?? c is a SyntaxError by spec design; write (a || b) ?? c. The ambiguity was considered dangerous enough to ban outright.
• Audit the ones you already shipped. Grep for || 0, || '', || false, and || <number> on config/option reads — each is a candidate for the exact bug above.

Resources

• Nullish coalescing operator (??) — MDN
• Logical OR (||) — MDN
• Nullish coalescing assignment (??=) — MDN
• Optional chaining (?.) — MDN
• TC39 proposal: nullish coalescing