discord.js/src/structures/interface/TextBasedChannel.js

/**
 * Interface for classes that have text-channel-like features
 * @interface
 */
class TextBasedChannel {

  constructor() {
    /**
     * A Map containing the messages sent to this channel.
     * @type {Map<String, Message>}
     */
    this.messages = new Map();
  }
  /**
   * Send a message to this channel
   * @param {String} content the content to send
   * @param {MessageOptions} [options={}] the options to provide
   * @returns {Promise<Message>}
   * @example
   * // send a message
   * channel.sendMessage('hello!')
   *  .then(message => console.log(`Sent message: ${message.content}`))
   *  .catch(console.log);
   */
  sendMessage(content, options = {}) {
    return this.client.rest.methods.sendMessage(this, content, options.tts);
  }
  /**
   * Send a text-to-speech message to this channel
   * @param {String} content the content to send
   * @returns {Promise<Message>}
   * @example
   * // send a TTS message
   * channel.sendTTSMessage('hello!')
   *  .then(message => console.log(`Sent tts message: ${message.content}`))
   *  .catch(console.log);
   */
  sendTTSMessage(content) {
    return this.client.rest.methods.sendMessage(this, content, true);
  }

  _cacheMessage(message) {
    const maxSize = this.client.options.max_message_cache;
    if (maxSize === 0) {
      // saves on performance
      return null;
    }

    if (this.messages.size >= maxSize) {
      this.messages.delete(Array.from(this.messages.keys())[0]);
    }

    this.messages.set(message.id, message);

    return message;
  }
}

function applyProp(structure, prop) {
  structure.prototype[prop] = TextBasedChannel.prototype[prop];
}

exports.applyToClass = (structure, full = false) => {
  const props = full ? ['sendMessage', 'sendTTSMessage', '_cacheMessage'] : ['sendMessage', 'sendTTSMessage'];
  for (const prop of props) {
    applyProp(structure, prop);
  }
};