js（canvas） 图片压缩

function compress(url, width, height) {
    return new Promise((resolve, reject) => {
        let img = document.createElement('img')
        img.onload = () => {
            let w = width
            let h = img.naturalHeight * (width / img.naturalWidth) //等比缩放
            if (h < height) {
                w = img.naturalWidth * (height / img.naturalHeight)
                h = height
            }
            let c = document.createElement('canvas')
            c.width = width
            c.height = height
            let ctx = c.getContext('2d')
            ctx.drawImage(img, 0, 0, w, h)
            resolve(c.toDataURL('image/png'))
        }
        img.onerror = reject
        img.src = url
    })
}