JavaScript - Nullish coalescing operator

The JavaScript nullish coalescing operator ?? returns right-hand side operand when left-hand side operand is null or undefined, otherwise returns left-hand side operand.

This can be contrasted with the logical OR || operator, which returns the right-hand side operand if the left operand is any falsy value (a value that is considered false in a Boolean context), not only null or undefined.

Example:

The example below shows how to use nullish coalescing operator.

const nullValue = null;
const someNumber = 50;

var valA = nullValue ?? "default for A";
var valB = someNumber ?? 0;

var txt;

txt = valA + "<br>";
txt = txt + valB + "<br>";

The output (value of txt) after running above script will be:

default for A
50

No chaining with AND or OR operators

It is not possible to combine both the AND (&&) and OR operators (||) directly with ??. In such cases, a SyntaxError will be thrown:

null || undefined ?? "foo"; // raises a SyntaxError
true || undefined ?? "foo"; // raises a SyntaxError

However, parenthesis can be used to explicitly indicate precedence.

(null || undefined) ?? "foo"; // returns "foo"