Conventions

Naming

The :: symbol is used inside AQL as the namespace separator. Using the namespace separator, users can create a multi-level hierarchy of function groups if required.

Examples:

RETURN myfunctions::myfunc()

RETURN myfunctions::math::random()

Note: As all function names in AQL, user function names are also case-insensitive.

Built-in AQL functions reside in the namespace _aql, which is also the default namespace to look in if an unqualified function name is found. Adding user functions to the _aql namespace is disallowed and will fail.

Variables and side effects

User functions can take any number of input arguments and should provide one result via a return statement. User functions should be kept purely functional and thus free of side effects and state, and state modification.

Modification of global variables is unsupported, as is changing the data of any collection from inside an AQL user function.

User function code is late-bound, and may thus not rely on any variables that existed at the time of declaration. If user function code requires access to any external data, it must take care to set up the data by itself.

All AQL user function-specific variables should be introduced with the var keyword in order to not accidentally access already defined variables from outer scopes. Not using the var keyword for own variables may cause side effects when executing the function.

Here is an example that may modify outer scope variables i and name, making the function not side effects-free:

function (values) {
  for (i = 0; i < values.length; ++i) {
    name = values[i];
    if (name === "foo") {
      return i;
    }
  }
  return null;
}

The above function can be made free of side effects by using the var or let keywords, so the variables become function-local variables:

function (values) {
  for (var i = 0; i < values.length; ++i) {
    var name = values[i];
    if (name === "foo") {
      return i;
    }
  }
  return null;
}

Input parameters

In order to return a result, a user function should use a return instruction rather than modifying its input parameters.

AQL user functions are allowed to modify their input parameters for input parameters that are null, boolean, numeric or string values. Modifying these input parameter types inside a user function should be free of side effects. However, user functions should not modify input parameters if the parameters are arrays or objects and as such passed by reference, as that may modify variables and state outside of the user function itself.

Return values

User functions must only return primitive types (i.e. null, boolean values, numeric values, string values) or aggregate types (arrays or objects) composed of these types. Returning any other JavaScript object type from a user function may lead to undefined behavior and should be avoided.

Enforcing strict mode

By default, any user function code will not be executed in strict mode or strong mode. In order to make a user function being run in strict mode, use use strict explicitly inside the user function, e.g.:

function (values) {
  "use strict"

  for (var i = 0; i < values.length; ++i) {
    var name = values[i];
    if (name === "foo") {
      return i;
    }
  }
  return null;
}

Any violation of the strict mode will trigger a runtime error.

Miscellaneous

Internally, user functions are stored in a system collection named _aqlfunctions of the selected database. That means that by default they are excluded from dumps created with arangodump. To include AQL user functions in a dump, the dump should be started with the option --include-system-collections true.