Method is

source/predicates/predicate.ts:232–240

Test if the value matches a custom validation function. The validation function should return `true` if the value passes the function. If the function either returns `false` or a string, the function fails and the string will be used as error message. @param validator - Validation function.

(validator: (value: T) => boolean | string)

	@param validator - Validation function.
	*/
	is(validator: (value: T) => boolean | string): this {
		return this.addValidator({
			message: (value, label, error) => (error
				? `(${label}) ${String(error)}`
				: `Expected ${label} \`${String(value)}\` to pass custom validation function`
			),
			validator,
		});
	}

	/**
	Use a custom validation function that throws an error when the validation fails. This is useful for reusing existing validators or composing complex validations.