error TS2536: Type 'string' cannot be used to index type 'T'

[BryanWilhite]

> tsc

src/utilities/array.utility.ts:48:41 - error TS2536: Type 'string' cannot be used to index type 'T'.

48             ) => ((accumulator[key] || (accumulator[key] = [])).push(current), accumulator),
                                           ~~~~~~~~~~~~~~~~

src/utilities/array.utility.ts:73:41 - error TS2536: Type 'number' cannot be used to index type 'T'.

73             ) => ((accumulator[key] || (accumulator[key] = [])).push(current), accumulator),
                                           ~~~~~~~~~~~~~~~~


Found 2 errors.

there are only 5 StackOverflow articles related to TS2536: https://stackoverflow.com/search?q=ts%282536%29

these @mariusschulz articles might be of help:

• https://mariusschulz.com/blog/mapped-types-in-typescript
• https://mariusschulz.com/blog/keyof-and-lookup-types-in-typescript

[BryanWilhite]

This is new as of 3.5.1, there was no error in 3.4.5.

microsoft/TypeScript#31661

[BryanWilhite]

Given a type variable T with a constraint C, when an indexed access T[K] occurs on the target side of a type relationship, index signatures in C are now ignored. This is because a type argument for T isn't actually required to have an index signature, it is just required to have properties with matching types.

microsoft/TypeScript#32704 (comment)

so, i have written grouping utilities based on generics, T extends { [key: string]: any }, and i think what the above is saying is that key stuff extending T is now ignored Typescript 3.5.x

[BryanWilhite]

this approach might be an alternative: https://gist.github.com/Mds92/053deeba7349ce59038d9980c5dba612

my gist shows how simple things can be by just using any and Function: https://stackblitz.com/edit/mds92-group-by?file=index.ts

not sure what version of Typescript is in use on StackBlitz

[BryanWilhite]

i think a solution is in place as tests ☔ are failing 🔥

ok, let me attempt to explain to my future self what has been done:

• ArrayUtility.groupBy replaces the previous two grouping functions
• ArrayUtility.groupBy returns an array of ReducedGroup, a new type to generalize grouping
• ReducedGroupUtility.reduceToObject<T> was added to transform an array of ReducedGroup to an object of the form, say, used by DisplayItemUtility.group

the tests for DisplayItemUtility.group are of course failing 🔬

[BryanWilhite]

it looks like ReducedGroupUtility.reduceToObject<T> is returning this unexpected shape:

console.log src/utilities/display-item.utility.ts:117
    group { reduced: { key: 'group-one', values: [ [Object], [Object] ] } }

[BryanWilhite]

more tests are passing:

console.log src/utilities/reduced-group-utility.ts:32
    { reduction: 
       { 'group-three': [ [Object] ],
         'group-two': [ [Object], [Object], [Object], [Object] ],
         'group-one': [ [Object], [Object] ] } }

[BryanWilhite]

my apologies to anyone out there getting a breaking change in a minor version: https://www.npmjs.com/package/songhay