app.ts

import * as ts from "typescript";

const fileName = "test.ts";
const program = ts.createProgram([fileName], {});
const typeChecker = program.getTypeChecker();
const file = program.getSourceFile(fileName);

// use getSignatureFromDeclaration
{
    const ctorDecl = ts.forEachChild(file, function find(n) {
        return (n.kind === ts.SyntaxKind.Constructor && <ts.ConstructorDeclaration>n) || ts.forEachChild(n, find);
    });
    writeParameters(typeChecker.getSignatureFromDeclaration(ctorDecl));
}
// use getSignaturesOfType
console.log("=========") 
{
    const classDecl = <ts.ClassDeclaration>file.statements[0];
    const constructorFunctionType =
        typeChecker.getTypeOfSymbolAtLocation(
            typeChecker.getSymbolAtLocation(classDecl.name),
            classDecl
        );
    writeParameters(typeChecker.getSignaturesOfType(constructorFunctionType, ts.SignatureKind.Construct)[0]);
}

function writeParameters(sig: ts.Signature) {
    for (const parameter of sig.getParameters()) {
        console.log(`${parameter.getName()}: ${typeChecker.typeToString(typeChecker.getTypeOfSymbolAtLocation(parameter, parameter.valueDeclaration))}`);
    }
}

test.ts

class A {
    constructor(a: number, b: string) {}
}