Conventions

Naming

Built-in AQL functions that are shipped with ArangoDB reside in the namespace _aql, which is also the default namespace to look in if an unqualified function name is found.

To refer to a user-defined AQL function, the function name must be fully qualified to also include the user-defined namespace. The :: symbol is used as the namespace separator. Users can create a multi-level hierarchy of function groups if required:

MYGROUP::MYFUNC()
MYFUNCTIONS::MATH::RANDOM()

Note: Adding user functions to the _aql namespace is disallowed and will fail.

User function names are case-insensitive like all function names in AQL.

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.

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-effect 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 (Function, Date, RegExp etc.) from a user function may lead to undefined behavior and should be avoided.

Enforcing strict mode

By default, any user function code will be executed in sloppy mode, not strict or strong mode. In order to make a user function 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.