Discord.js get an array of all messages in a channel

If you want to get a specific channel with an id, you need to do:

const channel = client.channels.cache.get("Your channel ID");

Then, to collect messages in this channel (limit is set to 100 messages max, so you can't collect more than 100 messages in this channel except if you do your own messages storing system).

channel.messages.fetch({ limit: 100 }).then(messages => {
  console.log(`Received ${messages.size} messages`);
  //Iterate through the messages here with the variable "messages".
  messages.forEach(message => console.log(message.content))
})

According to your title, you want all messages in a channel. Not just 100 of them. Since there is a max on number of messages you can fetch per request, we will have to use the before option to perform pagination to fetch all messages.

async function fetchAllMessages() {
  const channel = client.channels.cache.get("<my-channel-id>");
  let messages = [];

  // Create message pointer
  let message = await channel.messages
    .fetch({ limit: 1 })
    .then(messagePage => (messagePage.size === 1 ? messagePage.at(0) : null));

  while (message) {
    await channel.messages
      .fetch({ limit: 100, before: message.id })
      .then(messagePage => {
        messagePage.forEach(msg => messages.push(msg));

        // Update our message pointer to be last message in page of messages
        message = 0 < messagePage.size ? messagePage.at(messagePage.size - 1) : null;
      }
  }

  console.log(messages);  // Print all messages
}

As it is said in https://discord.js.org/#/docs/collection/master/class/Collection?scrollTo=array you can use the method <Collection>.array() to get an array of the values in the Collection. It is like [...Collection.values()] or Array.from(Collection.values()).