Get inferred type arguments from TypeScript compiler API

Question

I have the following type and function signature

type Ctor<T> = {
  new (): T
}

export function foo<T>(Ctor: Ctor<T>) {

}

class Bar { foo = 'what' }

foo(Bar) // inferred type of T is Bar

I trying to get the inferred type of T at the call site foo(Bar) using the compiler api. So far I've tried,

if (ts.isCallExpression(node)) { 
  const funcType = typeChecker.getTypeAtLocation(node.expression)
}

But this only gets the declared type of foo. It doesn't have the type arguments passed to it at the call site. I've also tried,

if (ts.isCallExpression(node)) { 
  const args = node.typeArguments
}

But this is also empty, I think because the types are not explicitly passed.

So how do I get the inferred type of T at each call site?

The question is not clear: the inferred type of T is the type of the argument you're passing to the function, so you already know it. Can you show a full minimal example of what you've tried, and what you'd like to get? — Blackhole
– Blackhole, Commented Sep 19, 2022 at 11:36
It's not the type of the argument, in this example, type of the argument is typeof Bar but T is type Bar. So I cant directly get T by checking the type of the argument. — Avin Kavish
– Avin Kavish, Commented Sep 19, 2022 at 11:42
@Blackhole OP is trying to get the inferred type of the foo call (which should be foo<Bar>(Bar)) through the compiler API. The two attempts they have shown aren't successful. — k.tten
– k.tten, Commented Sep 19, 2022 at 14:02
I just added the "typescript-compiler-api" tag, to better categorize the question and, I hope, to make it more obvious that this isn't just a regular typescript question — jcalz
– jcalz, Commented Sep 19, 2022 at 16:09

David Sherret · Accepted Answer · 2022-09-19 19:34:48Z

3

You can get this information from the resolved signature:

if (ts.isCallExpression(node)) {
  const signature = checker.getResolvedSignature(node);
  if (signature != null) {
    // outputs -- (Ctor: Ctor<Bar>): void
    console.log(checker.signatureToString(signature));
    const params = signature.getParameters();
    for (const param of params) {
      const type = checker.getTypeOfSymbolAtLocation(param, node);
      // outputs -- Ctor<Bar>
      console.log(checker.typeToString(type));
    }
  }
}

answered Sep 19, 2022 at 19:34

David Sherret

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