最近使用了 Canvas 2D https://developers.weixin.qq.com/miniprogram/dev/component/canvas.html https://developers.weixin.qq.com/miniprogram/dev/api/canvas/RenderingContext.html

这样一段代码，发现在本地运行是好的，但是总是有报警 Cannot read property 'scale' of undefined;at SelectorQuery callback function，这是为啥呢 canvas.getContext('2d') 文档也没说这个什么时候会获取不到 context

const query = this.createSelectorQuery();
query.select('#canvas-id')
    .fields({ node: true, size: true })
    .exec((res) => {
        if (res && res[0] && res[0].node) {
            const canvas = res[0].node;
            const ctx = canvas.getContext('2d');
            ctx.scale(2, 2);
        }
    });