Type narrowing

[swastik]

Hi there! Another question. We have a component typed liked this (rough code):

interface Args {
  type: 'a' | 'b'
}

{{#if (eq type 'a')}}
  <ComponentA @arg={{a}} />
{{/if}}

… and ComponentA is typed like this:

interface Args {
  type: 'a'
}

… I'll see an error saying ComponentA expects a and we're passing in 'a' | 'b' — is there a way to fix this? Thank you!

[chriskrycho]

You need an implementation of eq which "narrows" the same way that TypeScript's === does. At the moment, the types for eq from ember-truth-helpers or similar do not in general support that. I think we're missing some docs on that, but you can see how it works to give it those semantics in the implementing pull request. The reason this isn't "built in" is noted in the PR:

Note that the type effects of these operators do not strictly match the semantics specified by RFC 562, in that they express JavaScript truthiness instead of Handlebars truthiness. It's not clear to me that it is possible to express Handlebars truthiness in TypeScript in such a way that you can usefully narrow types with it in combination with logical operators.

In other words, it's the same basic scenario as discussed in #547!