[feature request] Better error messages for decorators (they are completely not understandable) · Microsoft/TypeScript#39991

(5 comments) (1 reaction) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import

BugDomain: Error MessagesEffort: ModerateHelp Wanted

Description

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

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.

Contributor guide

Tech stack: typescript
Domain: developer experience
Issue type: feature
Difficulty: 3
Estimated time: half day
Activity status: fresh
Clarity: clear
Prerequisites: understanding of TypeScript decoratorsfamiliarity with TypeScript compiler error reporting
Newbie friendliness: 60
Research direction: First, locate the error message 'TS1240: Unable to resolve signature of property decorator when called as an expression' in the TypeScript compiler source, likely in src/compiler/checker.ts or src/compiler/diagnosticMessages.json. Then, identify the specific condition that triggers this error (e.g., required parameter in decorator signature). Modify the error message to be more descriptive, such as 'The descriptor argument to a property decorator should not be required.' Consider adding more context about the expected signature. Test the change with existing test cases for decorators.