Why isn’t FeedMessage.decode.bind(FeedMessage) equivalent to x => FeedMessage.decode(x) in Javascript? [duplicate]

Below is a Javascript code which attempts to parse some protobuf input data.

'use strict';

import protobuf from 'protobufjs';

const proto = protobuf.loadSync('gtfs-realtime.proto');
const FeedMessage = proto.lookupType('transit_realtime.FeedMessage');

// sources is an array of Buffer containing the binary input
// const inputs = sources.map(FeedMessage.decode.bind(FeedMessage)); // this line does not work
const inputs = sources.map(x => FeedMessage.decode(x));

console.log(inputs[0].toJSON());

Why isn’t FeedMessage.decode.bind(FeedMessage) equivalent to x => FeedMessage.decode(x)?