Incorrect function type signature for ParameterDecorator

[bajtos]

TypeScript Version: 3.7.0-dev.20190905

Search Terms: ParameterDecorator propertyKey

Code

The type ParameterDecorator is defined as follows in src/lib/es5.d.ts:

TypeScript/src/lib/es5.d.ts

Line 1362 in 72bb4c2

declare type ParameterDecorator = (target: Object, propertyKey: string | symbol, parameterIndex: number) => void;

However, at runtime, constructor parameter decorators are invoked with propertyKey set to undefined.

Consider the following code:

function inject() {
  return function i(target: Object,
    propertyKey: string | symbol,
    parameterIndex: number
  ): void {
    console.log('propertyKey', typeof propertyKey);
  }
}

class FooBar {
  constructor(
    @inject()
    public name: string
  ) {}
}

Compile it with experimentalDecorators enabled in tsconfig.json.

Expected behavior:

Either:

• The code does not compile, because the decorator function must accept string | symbol | undefined in the propertyKey argument
• Or the program prints propertyKey string or propertyKey symbol at runtime

Actual behavior:

• The code compiles
• The programs prints undefined at runtime

Playground Link:

http://www.typescriptlang.org/play/index.html?experimentalDecorators=true#code/GYVwdgxgLglg9mABDMArAptAFASkQbwChFEAndKEUpUSWBZLKAQ1IHMKAuRAeQCMM0ADTESiAA6k449KSgBPANLp53AM5RSKNogA+iNfIC2fOABsRYia2ZGKsgJJgAJugAe3MCBOzRObgBucDDOBKIkEAhq5ugAdGZwbFgA5JLSsgrK8slCiAoycMASUjJySio4ANyiAL6EdYQQZsxqaogAYnBwAEKsYRFRmiDQcKRY4YgAAiiCULgT4iB8ZjAQiGC26Oqa2n4EdXVAA

Related Issues:

• Fix for #2249 #2635
• Constructor parameter decorator: "propertyKey" is undefined #20931
• Runtime parameter info (name, optional, default) #8126

[bajtos]

I find the current behavior problematic because the type definition for ParameterDecorator are not matching the actual runtime behavior.

I'd like to propose two different solutions for your consideration:

1. Change the ParameterDecorator type - add undefined to allowed propertyKey values.

   declare type ParameterDecorator = (target: Object, propertyKey: string | symbol | undefined, parameterIndex: number) => void; 

2. Modify the runtime to set propertyKey to an empty string '' instead of undefined when calling parameter decorators for constructor arguments.

Personally, I prefer the first one. I'd rather have my existing codebase to fail the compilation rather than discover a subtle change in TS runtime in production.

[jakebailey]

This was fixed back in TS 5.0 and is mentioned in the blog: https://devblogs.microsoft.com/typescript/announcing-typescript-5-0/#more-accurate-type-checking-for-parameter-decorators-in-constructors-under-experimentaldecorators