Why does bitwise “not 1” equal -2?

There are 2 integers between 1 and -2: 0 and -1

1   in binary is 00000000000000000000000000000001
0   in binary is 00000000000000000000000000000000
-1 in binary is 11111111111111111111111111111111
-2 in binary is 11111111111111111111111111111110
^{(“binary” being 2’s complement, in the case of a bitwise not ~ )}

As you can see, it’s not very surprising ~1 equals -2, since ~0 equals -1.

As @Derek explained, These bitwise operators treat their operands as a sequence of 32 bits. parseInt, on the other hand, does not. That is why you get some different results.

Here’s a more complete demo:

for (var i = 5; i >= -5; i--) {
  console.log('Decimal: ' + pad(i, 3, ' ') + '  |  Binary: ' + bin(i));
  if (i === 0)
    console.log('Decimal:  -0  |  Binary: ' + bin(-0)); // There is no `-0`
}

function pad(num, length, char) {
  var out = num.toString();
  while (out.length < length)
    out = char + out;
  return out
}

function bin(bin) {
  return pad((bin >>> 0).toString(2), 32, '0');
}

.as-console-wrapper { max-height: 100% !important; top: 0; }