How to call a function after a script is loaded? IONIC

In Ionic, I have two functions. The first should be called while the script is loading, and the second should be called after it.

  import * as testao from "src/assets/js/Script"
    
 async functionToCall(){
  await this.firstFunction();
  this.secondFunction();
   }
   async firstFunction() {
        let script = document.createElement('script');
        script.onload = () => {
          // perform actions you need on load of the script
          this.carts();
        }
        script.src = "https:***.js";
        script.onerror = () => {
        };
        document.body.appendChild(script);
      }
    
    carts() {
    testao.testFunc()
    }
    
    secondFunction(){
    }

I tried return Promise and async with await, but neither worked.