if message.content.startwith mention code example

Example: how to read if a person has send a message on discord.js

client.on('message', message => {
  // this function can check whether the content of the message you pass is the same as this message
  let filter = msg => {
    return msg.content.toLowerCase() == message.content.toLowerCase() && // check if the content is the same (sort of)
           msg.author == message.author; // check if the author is the same
  }

  message.channel.awaitMessages(filter, {
    maxMatches: 1, // you only need that to happen once
    time: 5 * 1000 // time is in milliseconds
  }).then(collected => {
    // this function will be called when a message matches you filter
  }).catch(console.error);
});