waitfor.ts

function waitFor( prop ) {
  return function( target, key, descriptor ) {
    var originalMethod = descriptor.value;
    descriptor.value = function (...args) {
        const intv = setInterval(() => {
            if (typeof window[prop] !== "undefined") {
                clearInterval(intv);
                originalMethod.apply(this, args);
            }    
        }, 50);
    };
    return descriptor;
  }
}

class Test {

    @waitFor("jQuery")
    public doStuff() {
        console.log((window as any).jQuery);
    }    

}

const test = new Test();
test.doStuff();

setTimeout(function(){
    (window as any).jQuery = function() {};
}, 1000);