问如何从一个js文件中回调另一个js文件？

EN

`var s = document.createElement("script");
s.setAttribute("src","js/Chart.min.js");
document.head.appendChild(s);

class BaseChart{

    constructor(area, settings) {

        this.area = area;
        this.settings = settings;
    }
    a() {
        new Chart(this.area, this.settings);
        console.log("BaseChart");
    }   
}


class BarChart extends BaseChart {
    constructor(area, settings) {
        super(area, settings);
    }
    /* constructor(ctx) {
        var data = {
            labels: ["January", "February", "March", "April", "May", "June", "July"],
            datasets: [
                {   
                    label: "My First dataset",
                    fill: false,
                    lineTension: 0.1,
                    backgroundColor: "rgba(75,192,192,0.4)",
                    borderColor: "rgba(75,192,192,1)",
                    borderCapStyle: 'butt',
                    borderDash: [],
                    borderDashOffset: 0.0,
                    borderJoinStyle: 'miter',
                    pointBorderColor: "rgba(75,192,192,1)",
                    pointBackgroundColor: "#fff",
                    pointBorderWidth: 1,
                    pointHoverRadius: 5,
                    pointHoverBackgroundColor: "rgba(75,192,192,1)",
                    pointHoverBorderColor: "rgba(220,220,220,1)",
                    pointHoverBorderWidth: 2,
                    pointRadius: 1,
                    pointHitRadius: 10,
                    data: [65, 59, 80, 81, 56, 55, 40],
                    spanGaps: false,
                },
                {
                    label: "Second",
                    data: [12, 56, 45, 23, 78, 23, 90],
                }
            ]
        };
        var lineChart = new Chart(ctx, {
            type: 'line',
            data: data
        });
    } */
    b() {
        super.a();
    }
}`