The | operator is overloaded for two types of operands: number and BigInt. For numbers, the operator returns a 32-bit integer. For BigInts, the operator returns a BigInt. It first coerces both operands to numeric values and tests the types of them. It performs BigInt OR if both operands become BigInts; otherwise, it converts both operands to 32-bit integers and performs number bitwise OR. A TypeError is thrown if one operand becomes a BigInt but the other becomes a number.

The operator operates on the operands' bit representations in two's complement. Each bit in the first operand is paired with the corresponding bit in the second operand: first bit to first bit, second bit to second bit, and so on. The operator is applied to each pair of bits, and the result is constructed bitwise.

The truth table for the OR operation is:

     9 (base 10) = 00000000000000000000000000001001 (base 2)
    14 (base 10) = 00000000000000000000000000001110 (base 2)
                   --------------------------------
14 | 9 (base 10) = 00000000000000000000000000001111 (base 2) = 15 (base 10)

Numbers with more than 32 bits get their most significant bits discarded. For example, the following integer with more than 32 bits will be converted to a 32-bit integer:

Before: 11100110111110100000000000000110000000000001
After:              10100000000000000110000000000001

For BigInts, there's no truncation. Conceptually, understand positive BigInts as having an infinite number of leading 0 bits, and negative BigInts having an infinite number of leading 1 bits.

Bitwise ORing any number x with 0 returns x converted to a 32-bit integer. Do not use | 0 to truncate numbers to integers; use Math.trunc() instead.

• Bitwise operators in the JS guide
• Bitwise OR assignment (|=)