typeargs.ts

import * as ts from "typescript";

const compilerOptions = {
  target: ts.ScriptTarget.Latest,
  module: ts.ModuleKind.ESNext,
  strict: true,
  noEmit: true,
  lib: [],
  types: [],
};

const code = `
    class A;
    function x<T>(a: T) {
        return "x";
    }

    function main() {
        // Here type parameter T is bound to type A.
        // I want to extract that information.
        x(new A());
    }
`;

const host = ts.createCompilerHost(compilerOptions);
host.getSourceFile = (fileName) => {
  if (fileName === "test.ts") {
    return ts.createSourceFile(fileName, code, ts.ScriptTarget.Latest);
  }
  return undefined;
};
const program = ts.createProgram(["test.ts"], compilerOptions, host);
const typeChecker = program.getTypeChecker();
const sourceFile = program.getSourceFile("test.ts");

function visit(node) {
  if (node.kind == ts.SyntaxKind.CallExpression) {
    const sig = typeChecker.getResolvedSignature(node);

    // Assert the function has type parameters
    console.assert(sig.declaration.typeParameters.length == 1);

    // I know the data I want is in `sig.mapper`, but that’s not a public API.

    // `typeParameters` is `undefined`
    console.log(sig.typeParameters);
    // `getTypeArguments` returns the empty array for `sig`, `node`, `node.expression`, etc
    const typeArg = typeChecker.getTypeArguments(sig);
    console.log({ typeArg });
    // What is the right API call here?
  }
  ts.forEachChild(node, visit);
}

visit(sourceFile);