Add XMP raw string type to ExpandedTags["xmp"] type

[suke]

Description

Fix tags.xmp._raw type to string.
Related to #183

Current behavior

Expected behavior

[mattiasw]

Thanks! Will merge at the same time as the other one.

[mattiasw]

Or actually, this can't make anyone's code start failing during type checking, right? It just expands what is correct.

[suke]

It probably won't fail the type checking.

However, I found a problem with the combination of intersection types and index signature.
This could be a problem when assigning properties to tags, but I don't think it's a problem if tags are read-only.
microsoft/TypeScript#17867

So far the assignment to tags.xmp seems to be working correctly.

ExifReader.load(file, { expanded: true }).then((tags) => {
  if(tags.xmp) {
    tags.xmp._raw= ''
    tags.xmp['Foo'] = {
      value: '',
      attributes: {},
      description: ''
    }
    tags.xmp['Bar'] = '' // Type 'string' is not assignable to type 'XmpTag'
  }
});

following code has the same problem.
https://github.com/mattiasw/ExifReader/blob/main/exif-reader.d.ts#L184

ExifReader.load(file, { expanded: false }).then((tags) => {
  tags['Foo'] = {
    description: '',
    value: 1
  }
});
// throw an error
// Type 'number' is not assignable to type 'string & XmpTag[] & XmpTags & String'.
// Type 'number' is not assignable to type 'string'.

[mattiasw]

Not sure I understand what the problem is. Both of the examples should fail type checking as I see it, and that should be the case even before your change in this PR.

[suke]

I've confirmed that the first example can't assign a string other than tags.xmp._raw.
This is correct behavior.

As you pointed out, the second exmaple is failing type checking before this PR.
If you do not intend to add properties to tags, there is no problem.

[0-CAT]

This change effectively makes it impossible to create tests/mocks based on XMP tags. After upgrading from 4.5.0, the following code will not compile due to missing _raw:

mockExifLoad.mockResolvedValueOnce({
  xmp: {
    description: { attributes: {}, description: "My description", value: null as never },
  },
});

However, adding _raw: "", to the xmp declaration also causes an error (which is the error discussed above):

I understand the idea that tags are supposed to be read-only, but it seems backwards to prevent people entirely from creating them because of the use of this weird edge-case of TypeScript unions.

Is this truly the intended behavior?

[mattiasw]

Hi! No, that was not intended. I gave it some time to try and fix it but I can't make it work. Maybe someone with a bit more TypeScript expertise knows how.

[0-CAT]

Yeah, I took a look as well. The link suke posted above was also very helpful. There are a few issues open on the TS project and even some proposals to properly handle this, but it hasn't yet come to a resolution.

• Need way to express hybrid types that are indexable for a subset of properties  microsoft/TypeScript#17867
• Allow any key type as an index signature parameter type microsoft/TypeScript#26797 & Negated types microsoft/TypeScript#29317
• How to define one property type different from any others?
• Property is not assignable to string index in interface
• How to define Typescript type as a dictionary of strings but with one numeric "id" property

It looks like the workaround is actually to do this the way it is, and then use an Object.assign hack to work around it. Something like this:

mockExifLoad.mockResolvedValueOnce({
  xmp: Object.assign({
    _raw: "",
  }, {
    description: { attributes: {}, description: "My description", value: null as never },
  } as XmpTags),
});

I guess we just have to hope for a future TypeScript version that gives us a better way to handle this.