4

I have the following type:

type MyType = {
    ['x']?: string;
    ['y']?: number;
} & { [key: string]: string };

When I write this:

const a: MyType = {
    'x': 'str',
    'y': 5
}

I get an error (y is a number while expecting a string) which makes sense according to the index signature. But when I write this:

const b: MyType = {};
b['x'] = 'str2';
b['y'] = 6

It works fine. What is the difference between the two? Should this even work?

Improve this question
4
  • 1
    See this question; the question here isn't exactly a duplicate, but the answer is related. microsoft/TypeScript#17867 might be the canonical issue about this, although there's no great answer in there from anyone who can definitively say if it "should" work to read/write properties whose type conflicts in the two intersection members. ms/TS#37809 is also related. Commented Mar 17, 2021 at 2:17
  • I feel like this is an issue with the ts compiler. I know that MyType shouldn't be written that way, This question is more about the ts compiler. For var a - it behaves like the index signature applies to everything - and you get a contradiction - and therefor an error. For var b - it behaves like the index signature applies to everything other than the x param and y param. so ts is ok with it and you don't get an error. (error for ts compiler) Sounds like a ts compiler bug - or I'm missing something. Commented Mar 17, 2021 at 16:36
  • 1
    Did you look at ms/TS#37809 linked above? Commented Mar 17, 2021 at 17:36
  • Oh Yeah, I missed that link Yeah, Thanks for the help - it is just a built in inconsistent behavior with ts Commented Mar 17, 2021 at 18:13
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