The logical OR assignment (||=) operator only evaluates the right operand and assigns to the left if the left operand is
.
Syntax
x ||= y
Description
Logical OR assignment short-circuits, meaning that x ||= y is equivalent to x || (x = y), except that the expression x is only evaluated once.
No assignment is performed if the left-hand side is not falsy, due to short-circuiting of the logical OR operator. For example, the following does not throw an error, despite x being const:
const x = 1;
x ||= 2;
Neither would the following trigger the setter:
const x = {
get value() {
return 1;
},
set value(v) {
console.log("Setter called");
},
};
x.value ||= 2;
In fact, if x is not falsy, y is not evaluated at all.
const x = 1;
x ||= console.log("y evaluated");
// Logs nothing
Examples
Setting default content
If the "lyrics" element is empty, display a default value:
document.getElementById("lyrics").textContent ||= "No lyrics.";
Here the short-circuit is especially beneficial, since the element will not be updated unnecessarily and won't cause unwanted side-effects such as additional parsing or rendering work, or loss of focus, etc.
Note: Pay attention to the value returned by the API you're checking against. If an empty string is returned (a
value), ||= must be used, so that "No lyrics." is displayed instead of a blank space. However, if the API returns null or
undefined in case of blank content, ??= should be used instead.