[feature request] Better error messages for decorators (they are completely not understandable)

[trusktr]

Search Terms

"Unable to resolve signature of property decorator when called as an expression"

Suggestion

The error message I received was error TS1240: Unable to resolve signature of property decorator when called as an expression.

This leaves absolutely no clue what is wrong.

Examples

To fix the issue, I had to change the signature of one of my functions from

export function attribute(
	prototype: any
	propName: string,
	descriptor: PropertyDescriptor | undefined,
): any

to

export function attribute(
	prototype: any
	propName: string,
	descriptor?: PropertyDescriptor | undefined,
): any

The difference is subtle: the descriptor arg isn't always passed to a decorator (depends on whether it is used on a property or on an accessor or method).

It'd be better if the message said something like The 'descriptor' argument to a property decorator should not be required.

Similar for other cases that trigger the same unhelpful error message.

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

[DanielRosenwasser]

Can you give a specific example of the usage of that function?

[a-tarasyuk]

I found two tests related to this error, however, maybe there is some specific case.

TypeScript/tests/cases/conformance/decorators/class/property/decoratorOnClassProperty7.ts

Lines 3 to 7 in 57e2fe0

	declare function dec(target: Function, propertyKey: string | symbol, paramIndex: number): void;
	class C {
	@dec prop;
	}

TypeScript/tests/cases/conformance/decorators/class/property/decoratorOnClassProperty6.ts

Lines 3 to 6 in 57e2fe0

	declare function dec(target: Function): void;
	class C {
	@dec prop;

[a-tarasyuk]

It seems duplicate of #4534

[trusktr]

As requested, here's a small reproduction on playground.

[Alexandrialexie]

It seems like this problem was resolved in v5.0.4. The error message gives more context now. It now reads:

Unable to resolve signature of property decorator when called as an expression.
The runtime will invoke the decorator with 2 arguments, but the decorator expects 3.(1240)

input.tsx(4, 2): An argument for 'descriptor' was not provided."

Are any more changes to the error message still needed?

Playground link