4 JavaScript preprocessing

Overview

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:

  1. return (value - 32)  * 5 / 9

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

  1. function (value)
  2.        {
  3.           return (value - 32) * 5 / 9
  4.        }

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:

  1. if (value == 0)
  2.            throw "Zero input value"
  3.        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 (https://duktape.org/) 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:

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

Examples

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 returned by the function.

Example 1: Convert a number (scientific notation to integer)

Convert the number “2.62128e+07” from scientific notation to an integer.

  1. return (Number(value))

Value returned by the function: 26212800.

Example 2: Convert a number (binary to decimal)

Convert the binary number “11010010” to a decimal number.

  1. return(parseInt(value,2))

Value returned by the function: 210.

Example 3: Round a number

Round the number “18.2345” to 2 digits.

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

Value returned by the function: 18.23.

Example 4: Count letters in a string

Count the number of letters in the string “Zabbix”.

  1. return (value.length)

Value returned by the function: 6.

Example 5: Get time remaining

Get the remaining time (in seconds) until the expiration date of a certificate (Feb 12 12:33:56 2022 GMT).

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

Value returned by the function: 44380233.

Example 6: Remove JSON properties

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

  1. var obj=JSON.parse(value);
  3.        for (i = 0; i < Object.keys(obj).length; i++) {
  4.            delete obj[i]["data_size"];
  5.            delete obj[i]["index_size"];
  6.        }
  8.        return JSON.stringify(obj)

Value accepted by the function:

  1. [
  2.            {
  3.                "table_name":"history",
  4.                "data_size":"326.05",
  5.                "index_size":"174.34"
  6.            },
  7.            {
  8.                "table_name":"history_log",
  9.                "data_size":"6.02",
  10.                "index_size":"3.45"
  11.            }
  12.        ]

Value returned by the function:

  1. [
  2.            {
  3.                "table_name":"history"
  4.            },
  5.            {
  6.                "table_name":"history_log"
  7.            }
  8.        ]
Example 7: Convert Apache status to JSON

Convert the value received from a web.page.get Zabbix agent item (e.g., web.page.get[http://127.0.0.1:80/server-status?auto\]) to a JSON object.

  1. // Convert Apache status to JSON
  3.        // Split the value into substrings and put these substrings into an array
  4.        var lines = value.split('\n');
  6.        // Create an empty object "output"
  7.        var output = {};
  9.        // Create an object "workers" with predefined properties
  10.        var workers = {
  11.            '_': 0, 'S': 0, 'R': 0, 'W': 0,
  12.            'K': 0, 'D': 0, 'C': 0, 'L': 0,
  13.            'G': 0, 'I': 0, '.': 0
  14.        };
  16.        // Add the substrings from the "lines" array to the "output" object as properties (key-value pairs)
  17.        for (var i = 0; i < lines.length; i++) {
  18.            var line = lines[i].match(/([A-z0-9 ]+): (.*)/);
  20.            if (line !== null) {
  21.                output[line[1]] = isNaN(line[2]) ? line[2] : Number(line[2]);
  22.            }
  23.        }
  25.        // Multiversion metrics
  26.        output.ServerUptimeSeconds = output.ServerUptimeSeconds || output.Uptime;
  27.        output.ServerVersion = output.ServerVersion || output.Server;
  29.        // Parse "Scoreboard" property to get the worker count
  30.        if (typeof output.Scoreboard === 'string') {
  31.            for (var i = 0; i < output.Scoreboard.length; i++) {
  32.                var char = output.Scoreboard[i];
  34.                workers[char]++;
  35.            }
  36.        }
  38.        // Add worker data to the "output" object
  39.        output.Workers = {
  40.            waiting: workers['_'], starting: workers['S'], reading: workers['R'],
  41.            sending: workers['W'], keepalive: workers['K'], dnslookup: workers['D'],
  42.            closing: workers['C'], logging: workers['L'], finishing: workers['G'],
  43.            cleanup: workers['I'], slot: workers['.']
  44.        };
  46.        // Return JSON string
  47.        return JSON.stringify(output);

Value accepted by the function:

  1. HTTP/1.1 200 OK
  2.        Date: Mon, 27 Mar 2023 11:08:39 GMT
  3.        Server: Apache/2.4.52 (Ubuntu)
  4.        Vary: Accept-Encoding
  5.        Content-Encoding: gzip
  6.        Content-Length: 405
  7.        Content-Type: text/plain; charset=ISO-8859-1
  9.        127.0.0.1
  10.        ServerVersion: Apache/2.4.52 (Ubuntu)
  11.        ServerMPM: prefork
  12.        Server Built: 2023-03-08T17:32:01
  13.        CurrentTime: Monday, 27-Mar-2023 14:08:39 EEST
  14.        RestartTime: Monday, 27-Mar-2023 12:19:59 EEST
  15.        ParentServerConfigGeneration: 1
  16.        ParentServerMPMGeneration: 0
  17.        ServerUptimeSeconds: 6520
  18.        ServerUptime: 1 hour 48 minutes 40 seconds
  19.        Load1: 0.56
  20.        Load5: 0.33
  21.        Load15: 0.28
  22.        Total Accesses: 2476
  23.        Total kBytes: 8370
  24.        Total Duration: 52718
  25.        CPUUser: 8.16
  26.        CPUSystem: 3.44
  27.        CPUChildrenUser: 0
  28.        CPUChildrenSystem: 0
  29.        CPULoad: .177914
  30.        Uptime: 6520
  31.        ReqPerSec: .379755
  32.        BytesPerSec: 3461.58
  33.        BytesPerReq: 3461.58
  34.        DurationPerReq: 21.2916
  35.        BusyWorkers: 2
  36.        IdleWorkers: 6
  37.        Scoreboard: ____KW__..............................................................................................................................................

Value returned by the function:

  1. {
  2.            "Date": "Mon, 27 Mar 2023 11:08:39 GMT",
  3.            "Server": "Apache/2.4.52 (Ubuntu)",
  4.            "Vary": "Accept-Encoding",
  5.            "Encoding": "gzip",
  6.            "Length": 405,
  7.            "Type": "text/plain; charset=ISO-8859-1",
  8.            "ServerVersion": "Apache/2.4.52 (Ubuntu)",
  9.            "ServerMPM": "prefork",
  10.            "Server Built": "2023-03-08T17:32:01",
  11.            "CurrentTime": "Monday, 27-Mar-2023 14:08:39 EEST",
  12.            "RestartTime": "Monday, 27-Mar-2023 12:19:59 EEST",
  13.            "ParentServerConfigGeneration": 1,
  14.            "ParentServerMPMGeneration": 0,
  15.            "ServerUptimeSeconds": 6520,
  16.            "ServerUptime": "1 hour 48 minutes 40 seconds",
  17.            "Load1": 0.56,
  18.            "Load5": 0.33,
  19.            "Load15": 0.28,
  20.            "Total Accesses": 2476,
  21.            "Total kBytes": 8370,
  22.            "Total Duration": 52718,
  23.            "CPUUser": 8.16,
  24.            "CPUSystem": 3.44,
  25.            "CPUChildrenUser": 0,
  26.            "CPUChildrenSystem": 0,
  27.            "CPULoad": 0.177914,
  28.            "Uptime": 6520,
  29.            "ReqPerSec": 0.379755,
  30.            "BytesPerSec": 1314.55,
  31.            "BytesPerReq": 3461.58,
  32.            "DurationPerReq": 21.2916,
  33.            "BusyWorkers": 2,
  34.            "IdleWorkers": 6,
  35.            "Scoreboard": "____KW__..............................................................................................................................................",
  36.            "Workers": {
  37.                "waiting": 6,
  38.                "starting": 0,
  39.                "reading": 0,
  40.                "sending": 1,
  41.                "keepalive": 1,
  42.                "dnslookup": 0,
  43.                "closing": 0,
  44.                "logging": 0,
  45.                "finishing": 0,
  46.                "cleanup": 0,
  47.                "slot": 142
  48.            }
  49.        }