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Here's is an interesting one:

if (a == 0 && a == 1 && a == 2) {
  console.log(a); // ?
}

JavaScript implicitly calls valueOf method to convert an object into primitive value whenever it needs to (see mdn).

So if you design your object this way...

const a = {
  value: 0,
  valueOf() {
    return this.value++;
  }
}

You got yourself a self-increment object : )