Canvas render image fail (?)

[qaqFei]

For examples(you should has "./test.png" file and "./images" folder):

const canvas = require('canvas');
const fs = require('fs');

async function main(){
    cv = canvas.createCanvas(500, 500);
    ctx = cv.getContext('2d');
    img = await canvas.loadImage('./test.png');
    width = 250;
    height = 250;
    cut = 0;
    for (;;) {
        width --;
        height --;
        cut ++;
        if (width < -5 || height < -5) {
            width = 250;
            height = 250;
        }
        ctx.clearRect(0, 0, 500, 500);
        ctx.drawImage(img, 0, 0, width, height);
        buf = cv.toBuffer('image/png');
        fs.writeFileSync(`./images/${cut}.png`, buf);
        console.log(width, height, cut);
    }
}

main();

if the width < 0 or height < 0, canvas.toBuffer will return null value(isnot "null").

how to fix?

add this code:

canvas.CanvasRenderingContext2D.prototype._drawImage = canvas.CanvasRenderingContext2D.prototype.drawImage
canvas.CanvasRenderingContext2D.prototype.drawImage = function(im, x, y, w, h){ //not perfect
    if (w < 0 || h < 0) return;
    this._drawImage(im, x, y, w, h);
}

but this is not perfect, because this function has other usage.

(I am a Chinese, so my English is not good.)