Type aliases don't work as object indexes - crash bug

[nickretallack]

If I make a file with only this in it:

type MyString = string;
type MyObject = {[id: MyString]: string};

And run the tool on it, this happens:

$ flow-to-ts myfile.js
error processing myfile.js
TypeError: Property name expected type of string but got null
    at Object.validate (/home/nretallack/.config/yarn/global/node_modules/@babel/types/lib/definitions/utils.js:159:13)
    at validateField (/home/nretallack/.config/yarn/global/node_modules/@babel/types/lib/validators/validate.js:24:9)
    at validate (/home/nretallack/.config/yarn/global/node_modules/@babel/types/lib/validators/validate.js:17:3)
    at builder (/home/nretallack/.config/yarn/global/node_modules/@babel/types/lib/builders/builder.js:38:27)
    at Object.TSTypeParameter (/home/nretallack/.config/yarn/global/node_modules/@babel/types/lib/builders/generated/index.js:1182:31)
    at indexers.forEach.indexer (/home/nretallack/.config/yarn/global/node_modules/@khanacademy/flow-to-ts/src/transform.js:564:35)
    at Array.forEach (<anonymous>)
    at Object.exit (/home/nretallack/.config/yarn/global/node_modules/@khanacademy/flow-to-ts/src/transform.js:554:16)
    at NodePath._call (/home/nretallack/.config/yarn/global/node_modules/@khanacademy/flow-to-ts/babel-traverse/lib/path/context.js:55:20)
    at NodePath.call (/home/nretallack/.config/yarn/global/node_modules/@khanacademy/flow-to-ts/babel-traverse/lib/path/context.js:42:17)

I suppose #108 didn't fix this.

[nickretallack]

It would be an improvement if it just assumed the type is always string, because object keys are always cast to string anyway, right?

[kevinbarabash]

I think we should try to convert the types as closely as possible. Changing a type alias to string seems like it'd be a pretty big change even if it is being used as a key.

[nickretallack]

Ultimately I did my typescript conversion using flowts. This guy maintains his own fork of babel-plugin-flow-to-typescript which is more up-to-date with bug fixes and support for new typescript 3.8 syntax. Also, flowts uses recast to avoid making unnecessary changes to your source code such as removing newlines and dropping comments.

[palexs]

This is still an issue when running flow-to-ts tool. However, when I use https://flow-to-ts.netlify.app/ it does the transformation correctly.

[jdelStrother]

This looks like it was introduced sometime around babel 7.6. If I add the following to my package.json:

  "resolutions": {
    "**/flow-to-ts/@babel/types": "< 7.6.0"
  }

flow-to-ts is able to convert indexer properties like this -

input:

// @flow
type KK = ("foo"|"bar")
export type X = { [k: KK]: string}

output:

type KK = "foo" | "bar";
export type X = { [k in KK]?: string };

[kevinbarabash]

@jdelStrother interesting. I've been meaning to update some of the dependencies. I'm glad to see doing so fixes some issues.

[jdelStrother]

Sorry, I wasn't clear. This used to work fine, but broke in Babel 7.6 or above 😞