The ?? (nullish coalescing) operator is a logical operator that returns its right-hand operand if the left-hand operand is null or undefined. Otherwise, it returns the left-hand operand.

Here’s a simple example:

const nullValue = null;
const digit = 0;

const foo = null ?? "default value";
const baz = digit ?? "default digit";

console.log(foo); // default value
console.log(baz); // 0

The main difference between the nullish coalescing operator (??) and the logical OR operator (||) is how they treat falsy values. The || operator returns the right-hand operand if the left-hand one is falsy (which includes 0, "", NaN, or false).

This can sometimes lead to unexpected results:

const count = 0;

const quantity = count || 42;

console.log(quantity); // 42 instead of 0

The ?? operator solves this issue by only considering null and undefined as invalid.

It’s perfect when you want to use a fallback value only when the first value is missing or undefined:

const count = 0;
const quantity = count ?? 42;

console.log(quantity); // 0

However, unlike ||, you can’t mix ?? directly with && or || without parentheses:

null || undefined ?? "foo"
'||' and '??' operations cannot be mixed without parentheses.

To fix this, group your expressions with parentheses:

(null || undefined) ?? "foo"

In summary, the ?? operator is a safer alternative to || when working with potential null or undefined values.