Logical UNDEF Operator

Overview

Logical Undef operator is a shortcut operator and this operator is like Logical OR operator of ||. When using || and LHS is false, the RHS will be evaluated. Compared to that, whe using ?? and LHS is null, the RHS will be evaluated.

The false meaning is wider than undefined/null, that is why the ?? is very useful especially when you distinguish between 0 and null.

var b = a ?? 0; // initializing 0 when a is uninitialized.

This can be also used with assignment operator as ??=.

a ??= new A();  // creating an instance only when a is uninitialized.

Examples

Example 1. Simple use

Code

function test(a) {
    return a ?? 10;
}

System.println(test(0));
System.println(test(1));
System.println(test(null));

Result

0
1
10

Example 2. With assignment

Code

var id;
class A() { @id = id++; }

function test(a) {
    a ??= new A();
    return a;
}

var a;
id = 10;
System.println((a = test(a)).id);
System.println((a = test(a)).id);
System.println((a = test(a)).id);

Result

10
10
10