如何在类外的另一个函数中使用类的方法？

这里有些错误，下面是代码的更好版本：

var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");

class Coordinates {
    compX(x) {
        var originX = myCanvas.width / 2;
        var mag = 20;
        return originX + mag * x;
    }
    compY(y) {
        var originY = myCanvas.height / 2;
        var mag = 20;
        return originY - mag * y;
    }
}

var xy = new Coordinates();

drawGraph(ctx, -10, 10);
drawAxis(ctx, 10);

function f(x) {
    return x * x;
}

function drawGraph(ctx, a, b) {
    //make lines that trace coordinates of the function in an interval
    var n = 50;
    var dx = (b - a) / n;
    for (x = a; x <= b; x += dx) {
        var pointAx = x;
        var pointAy = f(x);
        var pointBx = x + dx;
        var pointBy = f(x + dx);
        console.log(`[${pointAx}, ${pointAy}],[${pointBx}, ${pointBy}]`);

        ctx.moveTo(xy.compX(pointAx), xy.compY(pointAy)); //this is where the error occurs
        ctx.lineTo(xy.compX(pointBx), xy.compY(pointBy));
        ctx.stroke();
        ctx.strokeStyle = "#0000ff";
        ctx.lineWidth = 1;
    }
}

function drawAxis(ctx, axisLength) {
    ctx.strokeStyle = "#000000";
    ctx.moveTo(xy.compX(0), xy.compY(axisLength));
    ctx.lineTo(xy.compX(0), xy.compY(-axisLength));
    ctx.moveTo(xy.compX(axisLength), xy.compY(0));
    ctx.lineTo(xy.compX(-axisLength), xy.compY(0));
    ctx.stroke();
    ctx.lineWidth = 1;
}

首先，var/let/const在变量赋值之前习惯于写作。那是造成您所犯的一些错误的原因。第二，正如评论中提到的几个人所述，由于分配了之前drawGraph()调用的函数，因此出现了错误。将定义上移可以解决您的问题。xyxyxyxy unassigned

然后，不要像以前那样使用空的构造函数。如果构造函数中包含某些内容，请将其定义为constructor()而不是className()。