Key Mapping in Mapped Types

#12754

• "Key mapping" is really 3 features:

  • String concatenation types
  • A name clause for keys in mapped types
  • 4 new type operators - haven't done these

• String concat types

  • Thanks to recursive conditionals, you can now (cleanly) write a Join operator that takes a bunch of literals and separates them by a delimiter.
    • Join<["a", "b", "c"], "," -> "a,b,c"
    • You can write Split using inference; string splitting uses non-greedy matching.
      • Split<"a,b,c", ","> -> ["a", "b", "c"]

• Name clause for mapped types

  type Foo<T> = { [K in keyof T as `get_${K}` ]: () => T[K] };
  //                            ^^^^^^^^^^^^^
  // Current syntax

  • Naming breaks the special-casing for array-like types where it just maps over the indexes.
  • Interestingly, you can map to more or fewer keys in these clauses!
    • Map to never, and that removes a key
    • Map to multiple keys, and they'll all have the same type.
    • When multiple mapped keys overlap at any point, and they'll be unioned.
      I * Have to make sure that's the behavior (bug in current implementation)
      • This is currently already possible when you have the numeric key 0 and the string key "0".

• Operators: uppercase, lowercase, capitalize, uncapitalize

  • Locale?
    • Locale-neutral (?)
  • Eventually, people will want to go to/from kebab-case, camelCase, snake_case, etc.

• Previously had an error on large unions.

  • 100K constituents.
  • Had to enforce the same for string concatentation and spreads.
    • Can't do `${Digit}${Digit}${Digit}${Digit}${Digit}`

• Can imagine a prefix in these strings the same way find/replace works in editors

  • \u${SomeType}
  • But conflicts with strings
  • We like how we're not conflating the operations with the strings
  • Also, like [[something from C# - @rbuckton fill this in]].

• How do we feel about as in key mapping?

  • as looks like it is doing something with the constraint of the mapped type (the right side of the in)
  • Most other people on the team seem okay with it.

customDescriptionGenerator

https://github.com/microsoft/vscode-js-debug/blob/a04cff2e4c2b39616d4e1b813f41a40643b9fa76/OPTIONS.md#customdescriptiongenerator-1

• This is more about infrastructure for our team when debugging in VS Code.
• Ways to have nodes display a certain way in debuggers.
• Can add more information to Nodes, Symbols, Signatures, FlowNodes.
• Currently available through https://marketplace.visualstudio.com/items?itemName=ms-vscode.js-debug-nightly
  • https://github.com/microsoft/vscode-js-debug/blob/b5eac2827c2c16b64767c59bdeebb9da89fb60f5/README.md#nightly-extension
•