How to get a array's resolved type argument (tsc compiler API)

Question

I've already seen this other question, but although the name is similar, the question is really a different one, because they're trying to get the type of a property, which happens to be the same as the generic type argument, but I'm trying to get the actual resolved type argument, because I want to get the type T of Array<T>, and there's no property of type T inside the array type.

So I have a function like this:

import ts from 'typescript'

function convertType(tc: ts.TypeChecker, type: ts.Type) {
    if (type.symbol.name === 'Array') {
        debugger;
        // How do I get the resolved type of T?
    }
    else {
        //...
    }
}

I've noticed that if I debug the function and hover over type, it'll have a property called resolvedTypeArguments which has the exact type I need. So I could just do (type as any).resolvedTypeArguments, but that'd be a hack and I wonder if there's an official way to do it.

I was guessing I'd have to use one of the is... functions to cast the type into something that has a resolvedTypeArguments property, but there's no mention of "resolvedTypeArguments" anywhere in the whole "typescript.d.ts" file, so it seems that I'm not supposed to access that member at all.

Answer 1

You need to get the type of the first type parameter:

const params = checker.getTypeArguments(type);
console.log(checker.typeToString(params[0])); // T