babel/packages/babel-plugin-transform-nullish-coalescing-operator/test/fixtures/nullish-coalescing/runtime-semantics/exec.js

expect(null ?? undefined).toBeUndefined(undefined);
expect(undefined ?? null).toBeNull();
expect(false ?? true).toBe(false);
expect(0 ?? 1).toBe(0);
expect("" ?? "foo").toBe("");

var obj = { exists: true };
expect(obj.exists ?? false).toBe(true);
expect(obj.doesNotExist ?? "foo").toBe("foo");

var counter = 0;
function sideEffect() { return counter++; }
expect(sideEffect() ?? -1).toBe(0);

var counter2 = 0;
var obj2 = {
    get foo() { return counter2++; }
};
expect(obj2.foo ?? -1).toBe(0);

try {
  var access_a = 0;
  Object.assign(globalThis, { get a() { access_a++ } });
  a ?? {};
  expect(access_a).toBe(1);
} finally {
  delete globalThis.a;
}