req.* -> uppercase req.* as Get, Post etc...

req.getStatus -> req.Status

This is functional version for US:

var SEVERITY_COLORS = [
 '#97AAB3', '#7499FF', '#FFC859',
 '#FFA059', '#E97659', '#E45959'
];

var RESOLVE_COLOR = '#009900';

var SEND_MODE_HANDLERS = {
 alarm: handlerAlarm,
 event: handlerEvent
};

if (!String.prototype.format) {
 String.prototype.format = function() {
 var args = arguments;

return this.replace(/\{(\d+)}/g, function(match, number) {
 return number in args
 ? args[number]
 : match
 ;
 });
 };
}

function isEventProblem(params) {
 return params.event_value == 1
 && params.event_update_status == 0
 ;
}

function isEventUpdate(params) {
 return params.event_value == 1
 && params.event_update_status == 1
 ;
}

function isEventResolve(params) {
 return params.event_value == 0;
}

function getPermalink(mattermost_url, team_name, postid) {
 return '\{0}/\{1}/pl/\{2}'.format(
 mattermost_url.replace(/\/+$/, ''),
 team_name,
 postid
 );
}

function getChannel(send_to) {
 switch (true) {
 case /.+\/#.+/.test(send_to):
 return getChannelByName(send_to);

case /@.+/.test(send_to):
 return getDirectChannel(send_to);

default:
 return getChannelByID(send_to);
 }
}

function getChannelByName(send_to) {
 var team_chan = send_to
 .trim()
 .split('/#');

var resp = JSON.parse(req.Get(
 Mattermost.channel_byname.format(team_chan[0], team_chan[1]),
 JSON.stringify(fields)
 )
 );

if (req.Status() != 200) {
 throw '[\{0}] \{1}'.format(resp.status_code, resp.message);
 }

return resp;
}

function getDirectChannel(send_to) {
 Zabbix.Log(5, '[ Mattermost Webhook ] Call \{0}(\{1})'.format(
 arguments.callee.name,
 JSON.stringify(arguments)
 ));

var teamUser = send_to
 .trim()
 .split('/@'),
 bot = getBotUser(),
 user = getUserByName(teamUser[1]);

var resp = JSON.parse(req.Post(
 Mattermost.direct_channel,
 JSON.stringify([bot.id, user.id])
 )
 );

Zabbix.Log(5, '[ Mattermost Webhook ] Result \{0}: \{1}'.format(
 arguments.callee.name,
 JSON.stringify(resp)
 ));

if (req.Status() != 201) {
 throw '[\{0}] \{1}'.format(resp.status_code, resp.message);
 }

resp.team_name = teamUser[0];

return resp;
}

function getChannelByID(channelID) {
 Zabbix.Log(5, '[ Mattermost Webhook ] Call \{0}(\{1})'.format(
 arguments.callee.name,
 JSON.stringify(arguments)
 ));

var resp = JSON.parse(req.Get(
 Mattermost.get_channel.format(channelID),
 JSON.stringify(fields)
 )
 );

Zabbix.Log(5, '[ Mattermost Webhook ] Result \{0}: \{1}'.format(
 arguments.callee.name,
 JSON.stringify(resp)
 ));

if (req.Status() != 200) {
 throw '[\{0}] \{1}'.format(resp.status_code, resp.message);
 }

return resp;
}

function getBotUser() {
 Zabbix.Log(5, '[ Mattermost Webhook ] Call \{0}(\{1})'.format(
 arguments.callee.name,
 JSON.stringify(arguments)
 ));

var resp = JSON.parse(req.Get(
 Mattermost.bot_user,
 JSON.stringify(fields)
 )
 );

Zabbix.Log(5, '[ Mattermost Webhook ] Result \{0}: \{1}'.format(
 arguments.callee.name,
 JSON.stringify(resp)
 ));

if (req.Status() != 200) {
 throw '[\{0}] \{1}'.format(resp.status_code, resp.message);
 }

return resp;
}

function getUserByName(userName) {
 Zabbix.Log(5, '[ Mattermost Webhook ] Call \{0}(\{1})'.format(
 arguments.callee.name,
 JSON.stringify(arguments)
 ));

var resp = JSON.parse(req.Get(
 Mattermost.user_byname.format(userName),
 JSON.stringify(fields)
 )
 );

Zabbix.Log(5, '[ Mattermost Webhook ] Result \{0}: \{1}'.format(
 arguments.callee.name,
 JSON.stringify(resp)
 ));

if (req.Status() != 200) {
 throw '[\{0}] \{1}'.format(resp.status_code, resp.message);
 }

return resp;
}

function getTeamByID(teamID) {
 Zabbix.Log(5, '[ Mattermost Webhook ] Call \{0}(\{1})'.format(
 arguments.callee.name,
 JSON.stringify(arguments)
 ));

var resp = JSON.parse(req.Get(
 Mattermost.get_team.format(teamID),
 JSON.stringify(fields)
 )
 );

Zabbix.Log(5, '[ Mattermost Webhook ] Result \{0}: \{1}'.format(
 arguments.callee.name,
 JSON.stringify(resp)
 ));

if (req.Status() != 200) {
 throw '[\{0}] \{1}'.format(resp.status_code, resp.message);
 }

return resp;
}

function createProblemURL(zabbix_url, triggerid, eventid, event_source) {
 var problem_url = '';
 if (event_source === '0') {
 problem_url = '\{0}/tr_events.php?triggerid=\{1}&eventid=\{2}'
 .format(
 zabbix_url,
 triggerid,
 eventid
 );
 }
 else {
 problem_url = zabbix_url;
 }

return problem_url;
}

function getTagValue(event_tags, key) {
 var pattern = new RegExp('(' + key + ':.+)');
 var tagValue = event_tags
 .split(',')
 .filter(function (v) {
 return v.match(pattern);
 })
 .map(function (v) {
 return v.split(':')[1];
 })[0]
 || 0;

return tagValue;
}

function handlerAlarm(req, params) {
 var channel = getChannel(params.send_to);
 var fields = {
 channel_id: channel.id,
 props: {}
 };

if (isEventProblem(params)) {
 var team_name = channel.team_name
 ? channel.team_name
 : getTeamByID(channel.team_id).name;

fields.props.attachments = [
 createMessage(
 SEVERITY_COLORS[params.event_nseverity] || 0,
 params.event_date,
 params.event_time,
 createProblemURL(params.zabbix_url, params.trigger_id, params.event_id, params.event_source)
 )
 ];

var resp = JSON.parse(req.Post(
 Mattermost.post_message,
 JSON.stringify(fields)
 )
 );

if (req.Status() != 201) {
 throw '[\{0}] \{1}'.format(resp.status_code, resp.message);
 }

result.tags.__mattermost_post_id = resp.id;
 result.tags.__mattermost_channel_id = channel.id;
 result.tags.__mattermost_channel_name = channel.name;
 result.tags.__mattermost_message_link = getPermalink(
 params.mattermost_url,
 team_name,
 resp.id
 );

}
 else if (isEventUpdate(params)) {
 fields.root_id = getTagValue(params.event_tags, 'mattermost_post_id');

if (params.event_source === '0') {}
 fields.props.attachments = [
 createMessage(
 SEVERITY_COLORS[params.event_nseverity] || 0,
 params.event_update_date,
 params.event_update_time,
 createProblemURL(params.zabbix_url, params.trigger_id, params.event_id, params.event_source),
 true
 )
 ];

resp = JSON.parse(req.Post(
 Mattermost.post_message, JSON.stringify(fields)
 )
 );

if (req.Status() != 201) {
 throw '[\{0}] \{1}'.format(resp.status_code, resp.message);
 }

}
 else if (isEventResolve(params)) {
 fields.channel_id = getTagValue(params.event_tags, 'mattermost_channel_id');
 fields.id = getTagValue(params.event_tags, 'mattermost_post_id');
 fields.props.attachments = [
 createMessage(
 RESOLVE_COLOR,
 params.event_date,
 params.event_time,
 createProblemURL(params.zabbix_url, params.trigger_id, params.event_id, params.event_source)
 )
 ];

var post_id = getTagValue(params.event_tags, 'mattermost_post_id');

resp = JSON.parse(req.Put(
 Mattermost.chat_update.format(post_id),
 JSON.stringify(fields)
 )
 );

if (req.Status() != 200) {
 throw '[\{0}] \{1}'.format(resp.status_code, resp.message);
 }
 }
}

function handlerEvent(req, params) {
 var channel = getChannel(params.send_to);
 var fields = {
 channel_id: channel.id,
 props: {}
 };

if (isEventProblem(params)) {
 var team_name = channel.team_name
 ? channel.team_name
 : getTeamByID(channel.team_id).name;

fields.props.attachments = [
 createMessage(
 SEVERITY_COLORS[params.event_nseverity] || 0,
 params.event_date,
 params.event_time,
 createProblemURL(params.zabbix_url, params.trigger_id, params.event_id, params.event_source)
 )
 ];

var resp = JSON.parse(req.Post(Mattermost.post_message, JSON.stringify(fields)));

if (req.Status() != 201) {
 throw '[\{0}] \{1}'.format(resp.status_code, resp.message);
 }

result.tags.__mattermost_channel_name = channel.name;
 result.tags.__mattermost_message_link = getPermalink(
 params.mattermost_url,
 team_name,
 resp.id
 );

}
 else if (isEventUpdate(params)) {
 fields.props.attachments = [
 createMessage(
 SEVERITY_COLORS[params.event_nseverity] || 0,
 params.event_update_date,
 params.event_update_time,
 createProblemURL(params.zabbix_url, params.trigger_id, params.event_id, params.event_source),
 false
 )
 ];

resp = JSON.parse(req.Post(Mattermost.post_message, JSON.stringify(fields)));

if (req.Status() != 201) {
 throw '[\{0}] \{1}'.format(resp.status_code, resp.message);
 }

}
 else if (isEventResolve(params)) {
 fields.props.attachments = [
 createMessage(
 RESOLVE_COLOR,
 params.event_recovery_date,
 params.event_recovery_time,
 createProblemURL(params.zabbix_url, params.trigger_id, params.event_id, params.event_source)
 )
 ];

resp = JSON.parse(req.Post(Mattermost.post_message, JSON.stringify(fields)));

if (req.Status() != 201) {
 throw '[\{0}] \{1}'.format(resp.status_code, resp.message);
 }
 }
}

function createMessage(
 event_severity_color,
 event_date,
 event_time,
 problem_url,
 isShort
) {
 var message = {
 fallbac: params.alert_subject,
 title: params.alert_subject,
 color: event_severity_color,
 title_link: problem_url,
 footer: problem_url,

fields: [
 {
 title: 'Host',
 value: '\{0} [\{1}]'.format(params.host_name, params.host_ip),
 short: true
 },
 {
 title: 'Event time',
 value: '\{0} \{1}'.format(event_date, event_time),
 short: true
 }
 ],
 };


 if (params.event_source === '0') {
 message.fields.push(
 {
 title: 'Severity',
 value: params.event_severity,
 short: true
 },
 {
 title: 'Opdata',
 value: params.event_opdata,
 short: true
 }
 );
 }

if (!isShort && params.event_source === '0') {
 message.fields.push(
 {
 title: 'Event tags',
 value: '`\{0}`'.format(params.event_tags.replace(/__.+?:(.+?,|.+)/g, '') || 'None'),
 short: true
 },
 {
 title: 'Trigger description',
 value: params.trigger_description,
 short: true
 }
 );
 }

if (params.event_source !== '0' || params.event_update_status === '1') {
 message.fields.push(
 {
 title: 'Details',
 value: params.alert_message,
 short: false
 }
 );
 }

return message;
}

function validateParams(params) {
 if (typeof params.bot_token !== 'string' || params.bot_token.trim() === '') {
 throw 'Field "bot_token" cannot be empty';
 }

if (isNaN(params.event_id)) {
 throw 'Field "event_id" is not a number';
 }

if ([0, 1, 2, 3].indexOf(parseInt(params.event_source)) === -1) {
 throw 'Incorrect "event_source" parameter given: "' + params.event_source + '".\nMust be 0-3.';
 }

if (params.event_source !== '0') {
 params.event_nseverity = '0';
 params.event_severity = 'Not classified';
 params.event_update_status = '0';
 params.send_mode = 'event';
 }

if (params.event_source === '1' || params.event_source === '2') {
 params.event_value = '1';
 }

if (params.event_source === '1') {
 params.host_name = params.discovery_host_dns;
 params.host_ip = params.discovery_host_ip;
 }

if ([0, 1, 2, 3, 4, 5].indexOf(parseInt(params.event_nseverity)) === -1) {
 throw 'Incorrect "event_nseverity" parameter given: ' + params.event_nseverity + '\nMust be 0-5.';
 }

if (typeof params.event_severity !== 'string' || params.event_severity.trim() === '') {
 throw 'Field "event_severity" cannot be empty';
 }

if (params.event_update_status !== '0' && params.event_update_status !== '1') {
 throw 'Incorrect "event_update_status" parameter given: ' + params.event_update_status + '\nMust be 0 or 1.';
 }

if (params.event_value !== '0' && params.event_value !== '1') {
 throw 'Incorrect "event_value" parameter given: ' + params.event_value + '\nMust be 0 or 1.';
 }

if (typeof params.host_ip !== 'string' || params.host_ip.trim() === '') {
 throw 'Field "host_ip" cannot be empty';
 }

if (typeof params.host_name !== 'string' || params.host_name.trim() === '') {
 throw 'Field "host_name" cannot be empty';
 }

if (typeof params.mattermost_url !== 'string' || params.mattermost_url.trim() === '') {
 throw 'Field "mattermost_url" cannot be empty';
 }

if (!/^(http|https):\/\/.+/.test(params.mattermost_url)) {
 throw 'Field "mattermost_url" must contain a schema';
 }

if (['alarm', 'event'].indexOf(params.send_mode) === -1) {
 throw 'Incorrect "send_mode" parameter given: ' + params.send_mode + '\nMust be "alarm" or "event".';
 }

if (typeof params.send_to !== 'string' || params.send_to.trim() === '') {
 throw 'Field "send_to" cannot be empty';
 }

if (isNaN(params.trigger_id) && params.event_source === '0') {
 throw 'field "trigger_id" is not a number';
 }

if (typeof params.zabbix_url !== 'string' || params.zabbix_url.trim() === '') {
 throw 'Field "zabbix_url" cannot be empty';
 }

if (!/^(http|https):\/\/.+/.test(params.zabbix_url)) {
 throw 'Field "zabbix_url" must contain a schema';
 }

}

try {
 var params = JSON.parse(value);

validateParams(params);

var req = new CurlHttpRequest(),
 fields = {},
 result = \{tags: {}};

if (typeof params.HTTPProxy === 'string' && params.HTTPProxy.trim() !== '') {
 req.SetProxy(params.HTTPProxy);
 }

req.AddHeader('Content-Type: application/json; charset=utf-8');
 req.AddHeader('Authorization: Bearer ' + params.bot_token);

params.mattermost_url = params.mattermost_url.replace(/\/+$/, '');
 params.zabbix_url = params.zabbix_url.replace(/\/+$/, '');

var APIEndpoint = params.mattermost_url + '/api/v4/';

var Mattermost = {
 post_message: APIEndpoint + 'posts',
 get_channel: APIEndpoint + 'channels/\{0}',
 get_team: APIEndpoint + 'teams/\{0}',
 chat_update: APIEndpoint + 'posts/\{0}',
 direct_channel: APIEndpoint + 'channels/direct',
 channel_byname: APIEndpoint + 'teams/name/\{0}/channels/name/\{1}',
 user_byname: APIEndpoint + 'users/username/\{0}',
 bot_user: APIEndpoint + 'users/me'

};

params.send_mode = params.send_mode.toLowerCase();
 params.send_mode = params.send_mode in SEND_MODE_HANDLERS
 ? params.send_mode
 : 'alarm';

SEND_MODE_HANDLERS[params.send_mode](req, params);

if (params.event_source === '0') {
 return JSON.stringify(result);
 }
 else {
 return 'OK';
 }
}
catch (error) {
 Zabbix.Log(4, '[ Mattermost Webhook ] Mattermost notification failed: ' + error);
 throw 'Mattermost notification failed: ' + error;
}