class 在构造函数中添加事件监听器后属性变为未定义

Question

I am trying to create a class for drawing using typescript, but the property canvas becomes undefined if I add an event listener inside the constructor. here is my code:

export class Canvas {
    private canvas: HTMLCanvasElement;
    constructor() {
        this.canvas = document.getElementById("main_canvas") as HTMLCanvasElement;
        this.canvas.addEventListener("mousedown", this.method1)
    }
    method1() {
        let context = this.canvas.getContext("2d") as CanvasRenderingContext2D;
        context.moveTo(0, 0);
    }
}

here is the error:

Canvas.js:7 Uncaught TypeError: Cannot read properties of undefined (reading 'getContext')
    at HTMLCanvasElement.method1

Answer 1

You need to bind the callback. Either do:

this.canvas.addEventListener("mousedown", this.method1.bind(this))

or

this.canvas.addEventListener("mousedown", () => this.method1())

In your inital code, you set the callback to a function pointer ( this.method1 ), which will be executed with this refering to its current value in the executing context. In a click handler, that should be the clicked element, so this is not your object, but the HTML canvas element. And since the canvas has no property canvas , you get the error.

By binding the function with bind() , you get a new function, where this refers to whatever you put into bind.

In an arrow function, this always refers to what this is where the arrow function is declared.