Logical assignment

JavaScript course · Beginner · Lesson 92/133

Learn shortcut logical assignments with &&=, ||= and ??=.

Logical assignment

JavaScript offers three shortcuts that combine logical operators with assignment: &&=, ||= and ??=. They update a variable only when needed and keep code short.

x ||= 5 assigns 5 when x is falsy. x &&= 5 assigns 5 when x is truthy. x ??= 5 assigns 5 when x is null/undefined. Example: config.timeout ??= 3000;. If timeout already has a value it is kept; otherwise it becomes 3000.

When to use each

The three operators assign under different conditions, so you must choose the right one. ||= replaces falsy values, including 0, "" or false. ??= assigns only for null and undefined.

&&= assigns only when the variable is truthy; it is often used to conditionally update an object property. Example: item.inStock &&= true;. If item.inStock exists, it becomes true. Each operator fits a different intent.

Exercise Default with ||=

Set config to { retries: 0 }. Write config.retries ||= 3. Print config.retries.

Show example solution

const config = { retries: 0 };
config.retries ||= 3;
console.log(config.retries);

Exercise Combine all three assignments

Set state to { count: 0, label: null, mode: "on" }. Write state.count ||= 5. Write state.label ??= "Default". Write state.mode &&= "active". Print state.

Show example solution

const state = { count: 0, label: null, mode: "on" };
state.count ||= 5;
state.label ??= "Default";
state.mode &&= "active";
console.log(state);

Test yourself

Which operator assigns only for null/undefined?
- ??=
- ||=
- &&=
- =
Show answer

??= — Correct. ??= assigns only for null and undefined.

Assign with ??=

const config = { timeout: null };
config.timeout____ 3000;
console.log(config.timeout);

Show answer

const config = { timeout: null };
config.timeout??= 3000;
console.log(config.timeout);

Match operator to behavior

||=

assigns when falsy
??=

assigns when null/undefined
&&=

assigns when truthy
=

always assigns

Logical assignment