[] passes Record check

[timm-gs]

I have a type like the following specified:

const ConfigType = rt.Record({})
.And(rt.Partial({
  x: rt.String,
}));

Unfortunately running the following code succeeds without error:

ConfigType.check([])

[pelotom]

The corresponding static type failure is

type ConfigType = { x?: string }
const foo: ConfigType = []
//    ^^^ Type 'undefined[]' has no properties in common with type '{ x?: string; }'.

Are you saying you'd like a corresponding runtime check to this?

[timm-gs]

Yes, sorry I omitted that part. I'm using runtimes in this case to validate a config file loaded from JSON, and that's where it fails.

So the full sample code would be:

ConfigType.check(JSON.parse(`[]`))

[pelotom]

Just to be clear, though, this type of check would not flag an error if your ConfigType had any field that is present on arrays. For example:

type ConfigType = { x?: boolean, length: number }
const foo: ConfigType = []

This works in the static type system because ConfigType and Array are compatible and share a common property, length. If your goal is to outlaw the use of arrays for a ConfigType, even though they're technically compatible, you might want to use a Constraint. Note that the rt.Record({}).And(...) in your original example is superfluous, saying that something is an empty record is just saying that it could be literally anything except null or undefined, not precluding the possibility that it is an array.

[timm-gs]

Ok, so if this is by design I'll close this.

I have already added an extra check to verify that it's an object. So that seems settled then.