function to convert javascript integer to an array of 4 bytes

int32ToBytes.js

function int32ToBytes (int) {
  return [
    int & 0xff,
    (int >> 8) & 0xff,
    (int >> 16) & 0xff,
    (int >> 24) & 0xff
  ]
}

// 3 (base 10) = 00000000000000000000000000000011 (base 2)
console.log(int32ToBytes(3))  // -> [ 3, 0, 0, 0 ]

// 258 (base 10) = 00000000000000000000000100000010 (base 2)
console.log(int32ToBytes(258))  // -> [ 2, 1, 0, 0 ]

// 2147483647 (base 10) = 01111111111111111111111111111111 (base 2)
console.log(int32ToBytes(2147483647))  // -> [ 255, 255, 255, 127 ]

// -2147483648 (base 10) = 10000000000000000000000000000000 (base 2)
console.log(int32ToBytes(-2147483648)) // -> [ 0, 0, 0, 128 ]