@validate built-in decorator

The @validate decorator validates a value before it is set and silently rejects the write if the value is invalid. The function provided can throw to reject the write loudly.

const isPositive = n => n > 0;
class C {
	@validate(isPositive)
	accessor foo;
}

Decorator kinds

It applies to the following kinds of decorators:

• accessor
• setter

Later, it can also apply to these decorator extensions:

• let
• property

Using it on any other kind of decorator will throw a TypeError.

Other potential names

• assert
• check
• verify
• constraints

Implementation sketch

function validate (...validators) {
	if (validators.some(v => typeof v !== "function")) {
		throw new TypeError(`@validate arguments must be functions`);
	}

	const validator = function(value) {
		return validators.every(v => v.call(this, value));
	};

	return function (value, { kind, name }) {
		let set;
		const setter = function(newValue) {
			if (validator.call(this, newValue)) {
				return set.call(this, newValue);
			}
		};

		if (kind === "accessor") {
			set = value.set;
			value.set = setter;
			return value;
		}

		if (kind === "setter") {
			set = value;
			return setter;
		}

		throw new TypeError(`@validate cannot be used on ${kind} constructs`);
	};
};

Alternatives / extensions

• @validateAny() for the OR version?
• @validate.loud() version that throws when value is invalid.