JavaScript `||=`: Logical OR Assignment
1
min read .
Updated on
The logical OR assignment operator, ||=, assigns a new value only when the current left-hand value is falsy.
let name = '';
name ||= 'Guest';
console.log(name); // 'Guest'
Falsy values include false, 0, -0, 0n, '', null, undefined, and NaN.
Defaults in Functions
function greet(user) {
user ||= 'Stranger';
console.log(`Hello, ${user}!`);
}
greet('Alice'); // Hello, Alice!
greet(); // Hello, Stranger!
For ordinary function parameters, a default parameter is often clearer:
function greet(user = 'Stranger') {
console.log(`Hello, ${user}!`);
}Object Properties
const settings = { theme: '', fontSize: 14 };
settings.theme ||= 'light';||= vs. ??=
Use ||= when every falsy value should trigger the default. Use ??= when only null or undefined means “missing”:
let count = 0;
count ||= 10; // 10
let total = 0;
total ??= 10; // remains 0
That distinction is important for valid values such as 0, false, and the empty string.
Conclusion
||= is concise for assigning a fallback when the existing value is falsy. When zero, false, or empty strings are valid data, prefer ??= so those values are preserved.
