typescripttypescript-compiler-api

Get inferred type arguments from TypeScript compiler API


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?


Solution

  • 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));
        }
      }
    }