Custom validator configuration using Angular's public FieldContext API Returns ValidationError | ValidationError[] | null synchronously

Supports two patterns:

Function-based: { type: 'custom', functionName: 'myValidator' }
Expression-based: { type: 'custom', expression: 'fieldValue === formValue.password', kind: 'passwordMismatch' }

Properties

Name	Type	Description
errorParams	`Record<string, string> \| undefined`	Parameters to include in error object for message interpolation (expression-based pattern) Map of parameter names to expressions that will be evaluated in the same context as the validation expression
Example usage errorParams: { minValue: 'formValue.minValue', maxValue: 'formValue.maxValue' } // Allows message template: "Value must be between {{minValue}} and {{maxValue}}"
expression	`string \| undefined`	JavaScript expression to evaluate (expression-based pattern)
functionName	`string \| undefined`	Name of registered validator function (function-based pattern)
kind	`string \| undefined`	Error kind for expression-based validators - links to validationMessages
params	`Record<string, unknown> \| undefined`	Optional parameters to pass to validator function
type	`"custom"`	Validator type identifier
when inherited from `BaseValidatorConfig`	`ConditionalExpression \| undefined`	Conditional logic for when validator applies