Can a Variable Be Both 1 and 12? Exploring Impossible JS & Java Tricks

While browsing a community post, I encountered a seemingly impossible JavaScript snippet:

var a = ???;
if (a == 1 && a == 12) {
  console.log(a);
}

At first glance the condition looks absurd because a single variable cannot simultaneously equal two different values. However, the problem becomes intriguing when we consider that the variable might change its value dynamically during evaluation.

JS Version

One clever answer manipulates the valueOf or toString methods so that the variable returns different numbers on each comparison. The solution is illustrated in the image below.

Java Version

A Java counterpart achieves a similar effect by tampering with the internal integer cache:

Class cache = Integer.class.getDeclaredClasses()[0];
Field c = cache.getDeclaredField("cache");
c.setAccessible(true);
Integer[] array = (Integer[]) c.get(cache);
// array[129] is 1
array[130] = array[129]; // Set 2 to be 1
array[131] = array[129]; // Set 3 to be 1
Integer a = 1;
if (a == (Integer)1 && a == (Integer)2 && a == (Integer)3) {
   System.out.println("Success");
}

Another sophisticated answer uses PowerMockRunner to mock the integer cache, as shown in the following image.

Conclusion

The purpose of this article is not to exhaustively explore language internals, but to highlight how such puzzles can test one’s grasp of language features and encourage creative problem‑solving. They serve as both a fun challenge and a reminder of the depth hidden in everyday programming constructs.