Result of `Object.create(null)` may be passed to template literal

[erykpiast]

TypeScript Version: 3.5.1

Search Terms: template string, template literal, Object.create(null)

Code

`${Object.create(null)}`

Expected behavior: TypeError, as the object passed to the template string has no toString method in the prototype chain (as it has no prototype chain at all).

Actual behavior: Everything goes well in the compiler, but JavaScript reports runtime error (ex. TypeError: Cannot convert object to primitive value in Chrome).

Playground Link: https://www.typescriptlang.org/play/index.html#code/AYEg3g8gRgVgpgYwC4DoECc4EMlwBQB2ArgDYkCUAvsEA

Related Issues:
#30239
#1108

[erykpiast]

It should be possible, however, to pass that weird object-not-object thingy to tagged template literal, as tag function may be able to deal with that.

interface OptionalToString {
    toString?: () => string
}

function tag(_: TemplateStringsArray, value: OptionalToString) {
  if (typeof value.toString !== 'function') {
    return Object.prototype.toString.call(value);
  }

  return value.toString();
}

tag`${Object.create(null)}`;

See this in playground

[AnyhowStep]

Object.create(null) has bitten me in the past. I hate it. I don't like it when developers and libraries use it.

It's because of them, I need to do stuff like Object.prototype.hasOwnProperty.call(obj, k)

Because obj.hasOwnProperty fails for Object.create(null)

You just have to use libraries that actually support it and remember to support it, yourself.

It's not a TS problem

[fatcerberus]

Because obj.hasOwnProperty fails for Object.create(null)

Actually this is usually why you would use it: because you’re using an object as a dictionary and don’t want anything from Object.prototype interfering. Admittedly it’s less useful now that we have Map but it did serve a useful purpose when it was introduced.

If you ever wondered why we have a bunch of a static methods on Object now, e.g., Object.keys, rather than them being direct instance methods—this is why.