Added prefix command.

commands/prefix.js

+const errors = require('../errors.json');
+const config = require('../config.json');
+
+exports.run = async (client, message , args, connection) => {
+    if(!message.member.hasPermission('ADMINISTRATOR')) {
+            message.channel.send(errors.insufficient_permission_for_user).catch(() => {
+            return;
+        });
+    };
+    if (!args.join(' ')) {
+        message.channel.send(errors.insufficient_args).catch(() => {
+            return;
+        });
+        return;
+    };
+    const prefixdb = await new Promise((resolve, reject) => {
+        connection.query(`SELECT prefix FROM prefixes WHERE guild ='${message.guild.id}'`, function (err, result) {
+            if (err)
+                reject(err);
+            resolve(result);
+        });
+    });
+    var prefix;
+    if(prefixdb.length < 1) {
+        prefix = config.prefix;
+        } else {
+        prefix = prefixdb[0].prefix;
+    };
+    if (args[0] == "reset") {
+        if (prefix == config.prefix) {
+            message.channel.send('You didn\'t edited the prefix, so you can\'t reset.').catch(() => {
+                return;
+            });
+            return;
+        } else {
+            connection.query(`DELETE FROM prefixes WHERE guild ='${message.guild.id}'`, function (err) {
+                if (err) {
+                    throw err;
+                };
+            });
+            message.channel.send('Resetting...').catch(() => {
+                return;
+            });
+        };
+        return;
+    } else if (args[0] == prefix) {
+        message.channel.send(`Prefix is already ${prefix}`).catch(() => {
+            return;
+        });
+        return;
+    } else {
+        if (prefix == config.prefix) {
+            connection.query(`INSERT INTO prefixes (prefix, guild) VALUES ('${args[0]}', '${message.guild.id}')`, function (err) {
+                if (err) {
+                    throw err;
+                };
+                message.channel.send('Updated prefix.').catch(() => {
+                    return;
+                });
+            });
+        } else {
+            connection.query(`UPDATE prefixes SET prefix ='${args[0]}' WHERE guild ='${message.guild.id}'`, function (err) {
+                if (err) {
+                    throw err;
+                };
+            });
+            message.channel.send('Updated prefix.').catch(() => {
+                return;
+            });
+        };
+    };
+};
+
+module.exports.help = {
+    name: 'prefix',
+    aliases: ['set-prefix']
+};