fix(vega-typings): narrow type signature for vega-loader.http

[hydrosquall]

Motivation

• Providing a custom data fetcher, and noticed the response type could be narrowed.
• Checked vega-loader, I think it's already behaving as if this type is a Response rather than a string. (You can't use fetch directly, you have to wrap it so that it returns a string. See comment below for a usage example. )

References

• RequestInit: https://microsoft.github.io/PowerBI-JavaScript/interfaces/_node_modules_typedoc_node_modules_typescript_lib_lib_dom_d_.requestinit.html
• fetch: https://github.com/microsoft/TypeScript/blob/e96c10fe0ced0aa27ff40f0108df8e7153b348c7/lib/lib.dom.d.ts#L17373

• vega/packages/vega-loader/src/loader.js

  Lines 175 to 180 in 774165e

  	response = await fetch(url, opt);
  	return !response.ok
  	? error(response.status + '' + response.statusText)
  	: isFunction(response[type]) ? response[type]()
  	: response.text();

[hydrosquall]

I misspoke - it turns out you can't use fetch with loader.http directly, you have to wrap it in a function that returns a string. Something to this effect works as a default fetching function.

return fetch(url, options).then(response => {
            if (response.ok) {
                return response.text();
            }
            return response.text().then(text => {
                throw new Error(text);
            });
        });

[domoritz]

In

vega/packages/vega-loader/src/loader.js

Line 174 in 774165e

type = options && options.response,

, we read response, which I don't see in RequestInit. Are you sure the type is right?

[hydrosquall]

@domoritz, ah, good catch. I referring to this comment from the README.md, but you're right that RequestInit properties are not here.

From logging output at runtime,

this is where the options object is populated.

https://github.com/vega/vega/blob/02d79dddb2850a24abfe6a4ebaf8823548709bb5/packages/vega-dataflow/src/dataflow/load.js#L42-L4

The response type would be one of these, or text.

vega/packages/vega-loader/src/formats/index.js

Lines 7 to 11 in 02d79dd

	dsv: dsv,
	csv: delimitedFormat(','),
	tsv: delimitedFormat('\t'),
	json: json,
	topojson: topojson

Do you think the fix here would be to update the README for vega-loader (text copied below), update the implementation, or both?

If provided, the options argument may include any valid fetch RequestInit properties. The provided options will be combined with any default options passed to the loader constructor under the http property.

[domoritz]

For questions about the code and readme, @jheer knows best. The typings should follow the implementation.