Summary

Enables noUncheckedIndexedAccess across all code.

Details

Enabling the Typescript strict mode does not check for array index access (microsoft/TypeScript#39560) and assumes assessing a value at an array index produces the array/tuple type. However this can be deceiving and lead to errors in the code if this array access returns undefined.

Enabling this across the /src creates a consistent and more accurate type checking. This does require a bit more type guards to ensure the value is defined.

Note on tsconfig.json structures

So typescript/VScode changed how they respect nested tsconfig.json files. In the past, VScode would only look to the top-level tsconfig.json and ignore all other named-varients (i.e. tsconfig.lint.json) or nested tsconfig.json files (i.e. storybook/tsconfig.json).

Now this has changed, to where VScode will apply type checking using the nearest parent directory containing a tsconfig.json file. But the bad thing about this is that this assumes that tsconfig.json includes all child directories which may not be and often is not true! Thus making type checking a much bigger pain for it to be consistent between the IDE plugins and the tsc cli checks.

Ideally, we could apply certain compilerOptions to different directories and/or files much like eslint, because enabling noUncheckedIndexedAccess in test files is unnecessary and a little annoying.

To further complicate things, even if this did work, typescript automatically includes transitive imports thus requiring type checking on src code using different compilerOptions leading to conflicting linting errors like...

../packages/charts/src/chart_types/xy_chart/state/selectors/get_api_scale_configs.ts:93:9 - error TS2783: 'nice' is specified more than once, so this usage will be overwritten.

93         nice: false,
           ~~~~~~~~~~~

  ../packages/charts/src/chart_types/xy_chart/state/selectors/get_api_scale_configs.ts:96:9
    96         ...scaleConfigsByGroupId[groupId],
               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    This spread always overwrites this property.

Here spreading the value of scaleConfigsByGroupId[groupId] In one case is assumed to always be defined thus thinks that nice is always overridden, and the other case it's possibly undefined.

So with that said, the best solution is just to apply the same compilerOptions throughout the code and avoid nested overly-constrained tsconfig.json files, in favor of tsconfig.<something>.json.

The next idea is to allow forced unwrapping array access (i.e. someThing[0]!.someProperty) everywhere except /src. But there are no linting rules to enforce this, the only semi-related rule is @typescript-eslint/no-unnecessary-type-assertion

Issues

close #1757

Checklist

• The proper chart type label has been added (e.g. :xy, :partition)
• All related issues have been linked (i.e. closes #123, fixes #123)
• New public API exports have been added to packages/charts/src/index.ts
• Unit tests have been added or updated to match the most common scenarios