updatelight.js

this.buffer = Buffer.concat([
    // packet length is added by wrapper
    BufferMangler.writeVarInt(this.constructor.id), // 0x25
    BufferMangler.writeVarInt(this.x), // chunk x
    BufferMangler.writeVarInt(this.z), // chunk y
    BufferMangler.writeUInt8(1), // trust edges

    // sky light mask
    BufferMangler.writeVarInt(1),
    BufferMangler.writeBigUInt64BE(0b111111111111111110n), // Y=17 to 0

    // block light mask
    BufferMangler.writeVarInt(1),
    BufferMangler.writeBigUInt64BE(0b000000000000000000n), // Y=0

    // empty sky light mask
    BufferMangler.writeVarInt(1),
    BufferMangler.writeBigUInt64BE(0b000000000000000001n), // Y=-1

    // empty block light mask
    BufferMangler.writeVarInt(1),
    BufferMangler.writeBigUInt64BE(0b111111111111111111n), // Y=17 to -1

    // sky light array
    BufferMangler.writeVarInt(17), // length
    Buffer.concat(new Array(17).fill(0).map(() => {
        return Buffer.concat([
            BufferMangler.writeVarInt(2048),
            Buffer.concat(new Array(2048).fill(0xff).map(x => BufferMangler.writeUInt8(x)))
        ])
    })),

    // block light array
    BufferMangler.writeVarInt(0), // length
])