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Data Structures, Operators & Strings

Optional Chaining and Nullish Coalescing

?. stops a property lookup safely at the first null or undefined instead of throwing; ?? picks a default only for null or undefined.

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Definition

Optional chaining, `?.`, checks whether the value to its left is `null` or `undefined` before continuing the lookup. If it is, the whole expression short-circuits to `undefined` immediately, no error is thrown, and nothing further to the right is evaluated. If it is not, the chain continues as a normal property access or method call. This makes it safe to read a deeply nested path like `user?.address?.city` even when `address` might not exist, instead of writing a chain of manual `&&` checks or a try/catch. Nullish coalescing, `??`, returns its right-hand side only when the left-hand side is `null` or `undefined`, and returns the left-hand side unchanged for any other value, including falsy ones like `0`, `""`, `false`, and `NaN`. This is the key difference from `||`, which falls through to its right-hand side for any falsy value. `0 ?? 5` is `0`, but `0 || 5` is `5`. The two operators are often combined: `user?.settings?.volume ?? 100` safely reads a possibly-missing setting and only substitutes a default when it is genuinely absent.

Examples

const user = { address: null };
user.address.city;   // TypeError: Cannot read properties of null
user?.address?.city;  // undefined, no error

Optional chaining stops the lookup the moment it hits null, returning undefined instead of throwing.

0 ?? 5;   // 0 (0 is not null/undefined)
0 || 5;   // 5 (0 is falsy)
"" ?? "x"; // "" 
null ?? "x"; // "x"

?? only falls back for null or undefined; || falls back for any falsy value, which is why 0 and "" behave differently between the two.

const settings = { volume: 0 };
const volume = settings?.volume ?? 100;
// 0, not 100

Even though 0 is falsy, ?? treats it as a present value because it is not null or undefined, so the real setting wins over the default.

Where you see this

Practice this

Short-Circuit Default (||) (easy)Nullish Default (??) (easy)Optional Chaining (easy)

Further reading