Return undefined or null instead of throwing an exception

[aqueenan]

Motivation

Throwing an exception when returning JsValue::undefined() and JsValue::null() doesn't make sense because only a local exception handler could have any idea what it means.

https://eslint.org/docs/rules/no-throw-literal

Proposed Solution

Return undefined to JavaScript for JsValue::undefined() and return null to JavaScript for JsValue::null().

Also consider returning Symbol to JavaScript for JsValue::symbol(description: Option<&str>).

Alternatives

Instead of throwing value types, wrap them in a JavaScript Error object as recommended at https://eslint.org/docs/rules/no-throw-literal.

That's a good solution for other literals, so I'll raise it as another feature request.

However, for undefined and null, the JavaScript normal behaviour is to return undefined and null

[alexcrichton]

Thanks for the report @aqueenan! Can you perhaps show some example code for this issue? I think that the error object behavior looks to be covered by #1735 but I'm not sure what this issue means with respect to null and undefined. I'm also not entirely sure what you mean about Symbol values?

[alexcrichton]

From reading over the discussion in #1735 I think this was handled there, but let me know if I'm closing in error!