TypeScript typings fail to compile without downlevelIteration=true

[octogonz]

Describe the bug

The TypeScript typings for the yaml package fail to compile unless the downlevelIteration=true compiler option is enabled in the consuming project's tsconfig.json file.

To Reproduce

Attempt to compile some TypeScript code that does import * as yaml from 'yaml'; with downlevelIteration=false.

The error messages look like:

[typescript] Error: yaml/dist/nodes/Collection.d.ts:56:14 - (TS2569) Type 'Iterable<unknown>' is 
not an array type or a string type. Use compiler option '--downlevelIteration' to 
allow iterating of iterators.

The problem lines look like:

yaml/dist/nodes/Collection.d.ts

    /**
     * Removes a value from the collection.
     * @returns `true` if the item was found and removed.
     */
    deleteIn([key, ...rest]: Iterable<unknown>): boolean;
             ^^^^^^^^^^^^^^

Expected behaviour

We have this setting disabled in some of our projects that target ES5. The reason is that performance is very important, and transpilation of downlevelIteration was considered too expensive for those projects. Iterators are a nice language feature, but to ensure broadest compatibility with all potential consumers, a library package such as yaml should not rely on this feature in its API.

Note that I'm not insisting for yaml/**/*.js to adopt a particular transpilation strategy. This is merely a request that the .d.ts files should be declared in a way that will compile without errors.

Versions (please complete the following information):

• Environment: Node v14.15.4
• yaml: ~2.0.0-8
• TypeScript compiler: 4.2.4

[eemeli]

That's an entirely reasonable request, esp. as the current destructuring was leaking implementation details to the types.

@octogonz Would you have a way of verifying that the current master branch fixes your issues?

[eemeli]

Managed to first recreate the issue with yaml@2.0.0-8 and then verify that this issue is fixed with the change in ed5d1db.

This allows for compilation with the following TS compiler options:

{ target: 'es5', lib: ['es2015'], downlevelIteration: false }

[octogonz]

@eemeli I have confirmed that this issue is resolved in yaml@2.0.0-9. It works awesome! Thanks for the quick fix!