Union index signature parameter type has unclear / incorrect error

[catb0t]

TypeScript Version: 3.7.0-dev.20190913

Search Terms:

• index signature parameter type
• index signature union

Code

interface A {
  [id : string | number] : boolean;
};

Expected behavior:

Either the following error message is displayed, or the error message makes it clear that only one of string xor number is permissible as an index signature parameter type.

TypeScript/src/compiler/diagnosticMessages.json

Lines 962 to 965 in 038d951

	"An index signature parameter type cannot be a union type. Consider using a mapped object type instead.": {
	"category": "Error",
	"code": 1337
	},

Actual behavior:

error TS1023: An index signature parameter type must be 'string' or 'number'.

To me, the expression string | number is equivalent to the phrase "'string' or 'number'", but the phrasing is not precise.

Either the wrong error is being thrown, or this TS1023 should be clarified to say "...must be one of 'string' or 'number'." or "...must be either 'string' or 'number'."

TypeScript/src/compiler/diagnosticMessages.json

Lines 74 to 77 in 038d951

	"An index signature parameter type must be 'string' or 'number'.": {
	"category": "Error",
	"code": 1023
	},

Playground Link:
(error not shown in Playground)

Related Issues:

#2611
#2049
#8711
#18759

I will fix this in a PR if you tell me whether TS1023 is the right error with a fixed message, or if TS1337 should be thrown instead of TS1023.

[DanielRosenwasser]

I think you are correct that we should give that error message. Feel free to send a PR!

[fatcerberus]

An index signature parameter type cannot be a union type. Consider using a mapped object type instead.

I personally really like this one, as it's been my experience that this is generally what people are trying to do when they write such a signature. Alternatively, it could suggest using Record.

[DanielRosenwasser]

The current change is a good improvement and we can iterate based on feedback. Thanks @TroyTae!