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4 JavaScript preprocessing


This section provides details of preprocessing by JavaScript.

JavaScript preprocessing

JavaScript preprocessing is done by invoking JavaScript function with a single parameter 'value' and user provided function body. The preprocessing step result is the value returned from this function, for example, to perform Fahrenheit to Celsius conversion user must enter:

return (value - 32)  * 5 / 9

in JavaScript preprocessing parameters, which will be wrapped into a JavaScript function by server:

function (value)
          return (value - 32) * 5 / 9

The input parameter 'value' is always passed as a string. The return value is automatically coerced to string via ToString() method (if it fails then the error is returned as string value), with a few exceptions:

  • returning undefined value will result in an error
  • returning null value will cause the input value to be discarded, much like 'Discard value' preprocessing on 'Custom on fail' action.

Errors can be returned by throwing values/objects (normally either strings or Error objects).

For example:

if (value == 0)
           throw "Zero input value"
       return 1/value

Each script has a 10 second execution timeout (depending on the script it might take longer for the timeout to trigger); exceeding it will return error. A 64 megabyte heap limit is enforced.

The JavaScript preprocessing step bytecode is cached and reused when the step is applied next time. Any changes to the item's preprocessing steps will cause the cached script to be reset and recompiled later.

Consecutive runtime failures (3 in a row) will cause the engine to be reinitialized to mitigate the possibility of one script breaking the execution environment for the next scripts (this action is logged with DebugLevel 4 and higher).

JavaScript preprocessing is implemented with Duktape ( JavaScript engine.

See also: Additional JavaScript objects and global functions

Using macros in scripts

It is possible to use user macros in JavaScript code. If a script contains user macros, these macros are resolved by server/proxy before executing specific preprocessing steps. Note, that when testing preprocessing steps in the frontend, macro values will not be pulled and need to be entered manually.

Context is ignored when a macro is replaced with its value. Macro value is inserted in the code as is, it is not possible to add additional escaping before placing the value in the JavaScript code. Please be advised, that this can cause JavaScript errors in some cases.

In an example below, if received value exceeds a {$THRESHOLD} macro value, the threshold value (if present) will be returned instead:

var threshold = '{$THRESHOLD}';
       return (!isNaN(threshold) && value > threshold) ? threshold : value;


The following examples illustrate how you can use JavaScript preprocessing. Each example contains a brief description, a function body for JavaScript preprocessing parameters, and the preprocessing step result (value accepted by the function → value returned by the function).

Example 1: Convert number (scientific notation to integer)

Convert a number from scientific notation to an integer.

return (Number(value))

Result: 2.62128e+07 → 26212800

Example 2: Convert number (binary to decimal)

Convert a binary number to a decimal number.


Result: 11010010 → 210

Example 3: Round a number

Round a number to 2 digits.

return(Math.round(value* 100) / 100)

Result: 18.2345 → 18.23

Example 4: Count letters in a string

Count the number of letters in a string.

return (value.length)

Result: "zabbix" → 6

Example 5: Get time remaining

Get the remaining time (in seconds) until the expiration date of a certificate.

var split = value.split(' '),
           MONTHS_LIST = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'],
           month_index = ('0' + (MONTHS_LIST.indexOf(split[0]) + 1)).slice(-2),
           ISOdate = split[3] + '-' + month_index + '-' + split[1] + 'T' + split[2],
           now =;
       return parseInt((Date.parse(ISOdate) - now) / 1000);

Result: Feb 12 12:33:56 2022 GMT → 44380233

Example 6: Modify JSON (remove properties)

Modify a JSON data structure by removing any properties with the key "data_size" or "index_size".

var obj=JSON.parse(value);
       for (i = 0; i < Object.keys(obj).length; i++) {
           delete obj[i]["data_size"];
           delete obj[i]["index_size"];
       return JSON.stringify(obj)

Value accepted by the function:


Value returned by the function:

Example 7: Convert Apache status to JSON

Convert the value received from a Zabbix agent item (e.g.,[]) to a JSON object.

// Convert Apache status to JSON
       // Split the value into substrings and put these substrings into an array
       var lines = value.split('\n');
       // Create an empty object "output"
       var output = {};
       // Create an object "workers" with predefined properties
       var workers = {
           '_': 0, 'S': 0, 'R': 0, 'W': 0,
           'K': 0, 'D': 0, 'C': 0, 'L': 0,
           'G': 0, 'I': 0, '.': 0
       // Add the substrings from the "lines" array to the "output" object as properties (key-value pairs)
       for (var i = 0; i < lines.length; i++) {
           var line = lines[i].match(/([A-z0-9 ]+): (.*)/);
           if (line !== null) {
               output[line[1]] = isNaN(line[2]) ? line[2] : Number(line[2]);
       // Multiversion metrics
       output.ServerUptimeSeconds = output.ServerUptimeSeconds || output.Uptime;
       output.ServerVersion = output.ServerVersion || output.Server;
       // Parse "Scoreboard" property to get the worker count
       if (typeof output.Scoreboard === 'string') {
           for (var i = 0; i < output.Scoreboard.length; i++) {
               var char = output.Scoreboard[i];
       // Add worker data to the "output" object
       output.Workers = {
           waiting: workers['_'], starting: workers['S'], reading: workers['R'],
           sending: workers['W'], keepalive: workers['K'], dnslookup: workers['D'],
           closing: workers['C'], logging: workers['L'], finishing: workers['G'],
           cleanup: workers['I'], slot: workers['.']
       // Return JSON string
       return JSON.stringify(output);

Value accepted by the function:

HTTP/1.1 200 OK
       Date: Mon, 27 Mar 2023 11:08:39 GMT
       Server: Apache/2.4.52 (Ubuntu)
       Vary: Accept-Encoding
       Content-Encoding: gzip
       Content-Length: 405
       Content-Type: text/plain; charset=ISO-8859-1
       ServerVersion: Apache/2.4.52 (Ubuntu)
       ServerMPM: prefork
       Server Built: 2023-03-08T17:32:01
       CurrentTime: Monday, 27-Mar-2023 14:08:39 EEST
       RestartTime: Monday, 27-Mar-2023 12:19:59 EEST
       ParentServerConfigGeneration: 1
       ParentServerMPMGeneration: 0
       ServerUptimeSeconds: 6520
       ServerUptime: 1 hour 48 minutes 40 seconds
       Load1: 0.56
       Load5: 0.33
       Load15: 0.28
       Total Accesses: 2476
       Total kBytes: 8370
       Total Duration: 52718
       CPUUser: 8.16
       CPUSystem: 3.44
       CPUChildrenUser: 0
       CPUChildrenSystem: 0
       CPULoad: .177914
       Uptime: 6520
       ReqPerSec: .379755
       BytesPerSec: 3461.58
       BytesPerReq: 3461.58
       DurationPerReq: 21.2916
       BusyWorkers: 2
       IdleWorkers: 6
       Scoreboard: ____KW__..............................................................................................................................................

Value returned by the function:

           "Date": "Mon, 27 Mar 2023 11:08:39 GMT",
           "Server": "Apache/2.4.52 (Ubuntu)",
           "Vary": "Accept-Encoding",
           "Encoding": "gzip",
           "Length": 405,
           "Type": "text/plain; charset=ISO-8859-1",
           "ServerVersion": "Apache/2.4.52 (Ubuntu)",
           "ServerMPM": "prefork",
           "Server Built": "2023-03-08T17:32:01",
           "CurrentTime": "Monday, 27-Mar-2023 14:08:39 EEST",
           "RestartTime": "Monday, 27-Mar-2023 12:19:59 EEST",
           "ParentServerConfigGeneration": 1,
           "ParentServerMPMGeneration": 0,
           "ServerUptimeSeconds": 6520,
           "ServerUptime": "1 hour 48 minutes 40 seconds",
           "Load1": 0.56,
           "Load5": 0.33,
           "Load15": 0.28,
           "Total Accesses": 2476,
           "Total kBytes": 8370,
           "Total Duration": 52718,
           "CPUUser": 8.16,
           "CPUSystem": 3.44,
           "CPUChildrenUser": 0,
           "CPUChildrenSystem": 0,
           "CPULoad": 0.177914,
           "Uptime": 6520,
           "ReqPerSec": 0.379755,
           "BytesPerSec": 1314.55,
           "BytesPerReq": 3461.58,
           "DurationPerReq": 21.2916,
           "BusyWorkers": 2,
           "IdleWorkers": 6,
           "Scoreboard": "____KW__..............................................................................................................................................",
           "Workers": {
               "waiting": 6,
               "starting": 0,
               "reading": 0,
               "sending": 1,
               "keepalive": 1,
               "dnslookup": 0,
               "closing": 0,
               "logging": 0,
               "finishing": 0,
               "cleanup": 0,
               "slot": 142