ExtendedMessage.js

const { APIMessage, Structures } = require("discord.js");

class Message extends Structures.get("Message") {
    async inlineReply(content, options) {
        const mentionRepliedUser = typeof ((options || content || {}).allowedMentions || {}).repliedUser === "undefined" ? true : ((options || content).allowedMentions).repliedUser;
        delete ((options || content || {}).allowedMentions || {}).repliedUser;

        const apiMessage = content instanceof APIMessage ? content.resolveData() : APIMessage.create(this.channel, content, options).resolveData();
        Object.assign(apiMessage.data, { message_reference: { message_id: this.id } });
    
        if (!apiMessage.data.allowed_mentions || Object.keys(apiMessage.data.allowed_mentions).length === 0)
            apiMessage.data.allowed_mentions = { parse: ["users", "roles", "everyone"] };
        if (typeof apiMessage.data.allowed_mentions.replied_user === "undefined")
            Object.assign(apiMessage.data.allowed_mentions, { replied_user: mentionRepliedUser });

        if (Array.isArray(apiMessage.data.content)) {
            return Promise.all(apiMessage.split().map(x => {
                x.data.allowed_mentions = apiMessage.data.allowed_mentions;
                return x;
            }).map(this.inlineReply.bind(this)));
        }

        const { data, files } = await apiMessage.resolveFiles();
        return this.client.api.channels[this.channel.id].messages
            .post({ data, files })
            .then(d => this.client.actions.MessageCreate.handle(d).message);
    }
}

Structures.extend("Message", () => Message);