Built-in error prototypes are printed as "undefined" through Deno.inspect/console.log

[ghost]

All of the built-in error classes, besides Error, have prototype objects that show as "undefined" when logged, yet they're not actually undefined!

Example test code:

console.log(
  [
    Error,
    EvalError,
    RangeError,
    ReferenceError,
    SyntaxError,
    TypeError,
    URIError,
    AggregateError,
  ].map((ErrorClass) => ErrorClass.prototype),
);

Yet, observe that they're objects:

console.log(
  [
    Error,
    EvalError,
    RangeError,
    ReferenceError,
    SyntaxError,
    TypeError,
    URIError,
    AggregateError,
  ].map((ErrorClass) => typeof ErrorClass.prototype),
);

Very misleading, and threw me off while messing with the error prototypes :sweat_smile:

[andreubotella]

This seems to happen because TypeError.prototype instanceof Error returns true (and in general, SubClass.prototype instanceof ParentClass – this is not a bug, it's how JS works). This leads to the prototypes of native errors being treated as if they were errors in our console implementation:

https://github.com/denoland/deno/blob/3881e2e7bf3bbe4a5d62872a9fb7b6e2e3d82f81/ext/console/02_console.js#L1163-L1164

And inspectError prints the error's stack property followed by any causes, but the prototypes of native errors don't have a stack property, so it's undefined that gets printed.

[andreubotella]

Here's how the different browsers log the native error prototypes:

For regular errors, Chrome prints the stack, but for native error prototypes it seems to log them as error objects. The other browser engines do something specific to the native error prototypes.

[stale[bot]]

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