Typescript: Make the type guarding on 'typeof' be transitive.

[zheeeng]

TypeScript Version: 2.7.0

Search Terms:
typeof + transitive + type guard + typeof same

Code

function test(a: string | number, b: string | number) {
  if (typeof a === typeof b && typeof a === 'string') return b.length
}

Expected behavior:
variable b be asserted as string type.

Actual behavior:
TS report: Property 'length' does not exist on type 'string | number'. at b.length

Playground Link:
Playground

Related Issues:

[AlexGalays]

This could prove very useful.

Another real life example:

type FormStep = 'step1' | 'step2' | 'step3' ...

function view(step: FormStep, data: ...) {
   if (step === 'step1' && canAccessStep1(data.step1relatedStuff))
      return step1View(data.step1relatedStuff)

   if (step === 'step2' && canAccessStep2(data.step2relatedStuff))
     return step2View(data.step2relatedStuff)

   // The compiler can't infer that one anymore, because of the &&s above
   else
      return defaultView()
}

It seems like the same kind of limitations in control flow analysis; I can create a new ticket if it's not.

[zheeeng]

Any plan on this feature?