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) {
    // Assert the function has at least one parameter
    console.assert(node.arguments.length === 1);

    // Get the type of the argument passed to the function
    const argType = typeChecker.getTypeAtLocation(node.arguments[0]);

    // Get the type name
    const typeName = typeChecker.typeToString(argType);

    console.log(`Type of argument: ${typeName}`);
  }
  
  ts.forEachChild(node, visit);
}

visit(sourceFile);