Nullish Coalescing in JavaScript: What It Is and How It Works

The nullish coalescing operator (??) in JavaScript returns the right-hand value only if the left-hand value is null or undefined. It helps provide default values without treating other falsy values like 0 or '' as empty.

How It Works

Think of the nullish coalescing operator ?? as a smart helper that checks if a value is truly missing or empty in a special way. It only considers null or undefined as missing, unlike other operators that treat many values like 0, false, or empty strings as empty.

Imagine you ask a friend for their phone number. If they say "I don't have one" (which is like null or undefined), you use a backup number. But if they say "0" or "" (empty), you accept that as their real answer. This is how ?? works—it only switches to the backup if the first answer is truly missing.

Example

This example shows how ?? returns the left value unless it is null or undefined, then it returns the right value as a default.

javascript

const value1 = 0 ?? 42;
const value2 = null ?? 42;
const value3 = undefined ?? 42;
const value4 = '' ?? 'default';

console.log(value1); // 0
console.log(value2); // 42
console.log(value3); // 42
console.log(value4); // ''

Output

0
42
42

When to Use

Use nullish coalescing when you want to provide a default value only if a variable is null or undefined, but keep other valid falsy values like 0, false, or empty strings intact.

For example, when reading user input or configuration settings, you might want to accept 0 or false as valid inputs and only replace missing values with defaults. This avoids bugs where || would wrongly replace these falsy but valid values.

Key Points

• ?? only checks for null or undefined.
• It preserves other falsy values like 0, false, and empty strings.
• It helps avoid unintended defaulting when using ||.
• It is useful for setting default values safely.

Key Takeaways

Use ?? to provide defaults only when a value is null or undefined.

?? keeps valid falsy values like 0, false, and '' unchanged.

It prevents bugs caused by || treating all falsy values as empty.

Ideal for safely handling optional or missing data inputs.