Custom operators

It is possible to specify custom patch operators with the operators configuration property.

operators:
  __power:
    apply: (value ** (arg || 1))
    attribute: [number]
    argument: [number, empty]

The key (here __power) is the operator's name. The value is an object with four properties: apply, check, attribute and argument. Only apply is required, but attribute and argument are recommended.

apply property

apply is the function performing the transformation. It must return the new value after the transformation has been applied. Besides the regular parameters, the following parameters can be used:

• value: the current value of the attribute, i.e. before transformation
• arg: the argument passed to the patch operator
• type: the attribute's type, e.g. string or integer

attribute property

attribute is the list of attribute types that can use this operator.

For example, an __power operator can only be used on number attributes.

The possible values are: string, number, integer, boolean, any, string[], number[], integer[], boolean[] and any[].

argument property

argument is the list of possible types for the argument of this operator.

For example, a __power operator must take a single number value as argument. We might also allow an empty argument if there is a default argument value.

The possible values are the same as attribute with the following additional types: empty, empty[], object and object[].

If several arguments must be passed, it should be done by specifying an array argument. To specify an array argument with mixed types, use several array types. For example argument: ['number[]', 'string[]'] requires the argument to be an array containing either numbers, strings or a mix of them.

check property

check is a function applied to validate the argument of the patch operator. It should be used if the argument property is not flexible enough, e.g. if the argument is required to be a positive integer.

If the validation succeeds, it should not return anything. Otherwise, it should return the error message as a string.

The same parameters as apply are available, with the exception of value.

Throwing errors

Neither the apply nor the check property should throw errors. Use the attribute, argument and check properties for validation instead.

Empty values

When defining the apply and check properties, remember that:

• the value parameter might be undefined, unless the attribute is a required attribute.
• the operator's argument can only be null if the argument property is omitted or if it specifies empty. When the argument is empty, the arg parameter will be undefined.