What bit manipulation tricks are useful in interviews?

Bit operations work directly on the binary representation. They are fast, but they are also easy to get wrong with signs and fixed-width integers. In JavaScript, bitwise operators coerce operands to 32-bit signed integers, which matters for large values.

function singleNumber(values) {
  let result = 0;
  for (const value of values) result ^= value;
  return result;
}

function countSetBits(value) {
  let count = 0;
  while (value > 0) {
    value &= value - 1;
    count += 1;
  }
  return count;
}

function isSet(value, position) {
  return (value & (1 << position)) !== 0;
}

The core identities

• x ^ x = 0 and x ^ 0 = x, so XOR finds the value that appears once
• x & (x - 1) clears the lowest set bit
• x & -x isolates the lowest set bit
• x ^ (1 << k) toggles bit k
• x >> 1 divides by two for non-negative integers, while >>> shifts unsigned

Subsets can be enumerated with a bitmask from 0 to (1 << n) - 1. That is useful for small n, but the exponential bound is real. Do not use a bitmask when n is large enough that 2^n is impractical.