Acording to the documentation 'binary' is an alias for 'latin1'

If you want a binary representation the easiest way to get a string representation of the buffer with only 1 and 0 is to convert the buffer to hexadecimal his representation, then parse this representation into a BigInt, get the base 2 of the BigInt, and finally pad the string with non significant zeros

function buf2bin (buffer) {
  return BigInt('0x' + buffer.toString('hex')).toString(2).padStart(buffer.length * 8, '0')
}

If what you're asking is you want a string representation (expressed in binary) of each separate byte in the Buffer all combined into one string with space separators for each byte, then you can do that like this:

function convertBufferToBinaryString(buf, len) {
    let result = [];
    for (let i = 0; i < len; i++) {
        result.unshift(buf[i].toString(2).padStart(8, "0"));
    }
    return result.join(" ");
}

// sample usage

let buff = Buffer.alloc(10);
buff[0] = 10;
buff[1] = 16;
buff[2] = 20;
buff[3] = 32;
buff[4] = 9;

let str = convertBufferToBinaryString(buff, 5);
console.log(str);

Since this is a custom output format, there is no built-in way to do this. You can convert any single number to a binary string representation using .toString(2), but to combine the binary string representation of each separate byte in the buffer, you would have to build that combined result yourself (as shown above).

Note: This implementation assumes you want the [0] element of the buffer at the right end of the result string. That's why it uses .unshift() when building the result array. If you want the [0] element at the left end of the string, then use .push() instead.

Simple reduce to string with padStart combination.

const buff = Buffer.alloc(10);
buff[0] = 10;
buff[1] = 16;
buff[2] = 20;
buff[3] = 32;
buff[4] = 9;

const paddedBinString = buff.reduce(
    (acc, byte) => (acc += byte.toString(2).padStart(8, "0")),
    ""
);

console.log(paddedBinString);
// 00001010 00010000 00010100 00100000 00001001 00000000 00000000 00000000 00000000 00000000