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# 4 Preelaborazione JavaScript

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### Panoramica

Questa sezione fornisce dettagli sulla pre-elaborazione da parte di JavaScript.

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#### 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:

``` {.java}
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:

``` {.java}
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
(<https://duktape.org/>) JavaScript engine.

See also: [Additional JavaScript objects and global
functions](/manual/config/items/preprocessing/javascript/javascript_objects)

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##### 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.

::: noteclassic
 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:

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

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### 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 accepted by the function → value returned by the function).

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

Convert a number from scientific notation to an integer.

```javascript
return (Number(value))
```

Result: 2.62128e+07 → 26212800

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##### Example 2: Convert number (binary to decimal)

Convert a binary number to a decimal number.

```javascript
return(parseInt(value,2))
```

Result: 11010010 → 210

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##### Example 3: Round a number

Round a number to 2 digits.

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

Result: 18.2345 → 18.23

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##### Example 4: Count letters in a string

Count the number of letters in a string.

```javascript
return (value.length)
```

Result: "zabbix" → 6

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##### Example 5: Get time remaining

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

```javascript
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 = Date.now();

return parseInt((Date.parse(ISOdate) - now) / 1000);
```

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

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##### Example 6: Modify JSON (remove properties)

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

```javascript
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:

```json
[
    {
        "table_name":"history",
        "data_size":"326.05",
        "index_size":"174.34"
    },
    {
        "table_name":"history_log",
        "data_size":"6.02",
        "index_size":"3.45"
    }
]
```

Value returned by the function:

```json
[
    {
        "table_name":"history"
    },
    {
        "table_name":"history_log"
    }
]
```

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##### Example 7: Convert Apache status to JSON

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

```javascript
// 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];

        workers[char]++;
    }
}

// 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

    127.0.0.1
    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:

```json
{
    "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
    }
}
```

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