Mastering Default Values in JavaScript with the ?? Operator

the nullish coalescing (??) operator

Ever set a default value in JavaScript like this?

const port = config.port || 3000;

Seems fine, right? But what if config.port is 0? Oops — that’s a valid value for a port, but it gets replaced by 3000 because 0 is considered falsy. 😬

That’s where the nullish coalescing operator (??) steps in — a modern JavaScript feature that helps us assign defaults more safely and predictably.

So What’s the Problem with ||?

The logical OR (||) operator is often used to assign default values:

const username = userInput || 'Guest';

It works great unless the value you’re checking is a valid falsy one — like:

• 0
• false
• "" (empty string)
• NaN

Example:

const score = 0;
const defaultScore = 10;

console.log(score || defaultScore); // 10 ❌ (oops - 0 is valid!)

Here, 0 is treated as falsy, so the fallback kicks in — even though 0 might be the value you wanted to keep.

Meet the ?? Operator

The nullish coalescing (??) operator works similarly to ||, but with one key difference:

It only considers null or undefined as “missing” values.

Example:

const score = 0;
const defaultScore = 10;

console.log(score ?? defaultScore); // ✅ 0 - preserved!

That’s because 0 is not null or undefined, so ?? keeps it.

Why ?? Is a Safer Bet

• Preserves valid falsy values like 0, false, and ""
• Avoids accidental overwrites with defaults
• Helps make your code more predictable and readable

If you’re writing form logic, config defaults, or handling optional parameters — ?? is your new best friend.

Bonus Tip: Don’t Mix ?? and || Without Parentheses

// This throws an error!
const result = null || undefined ?? 'default';

Wrap expressions if you’re mixing them:

const result = (null || undefined) ?? 'default';

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