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# 4 JavaScript preprocessing

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#### Overview

This section provides details of preprocessing by 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 (10MB before Zabbix
5.0.9).

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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