demo/typescript.js
function getTypeOfPropertyOfType(type, name) {
            var prop = getPropertyOfType(type, name);
            return prop ? getTypeOfSymbol(prop) : undefined;
        }
function getTypeOfPropertyOfType(type, name) {
            var prop = getPropertyOfType(type, name);
            return prop ? getTypeOfSymbol(prop) : undefined;
        }
Return the type of the given property in the given type, or undefined if no such property exists