思路：

1. 将图片url转为Image对象
2. 将Image对象绘制到Canvas上
3. 将Canvas对象转为Base64

实现代码

// Image对象转base64
function imageToBase64(image) {
    let canvas = document.createElement('canvas')
    let width = image.width
    let height = image.height

    canvas.width = width
    canvas.height = height
    let context = canvas.getContext('2d')
    context.drawImage(image, 0, 0, width, height)
    return canvas.toDataURL('image/png')
}

// 回调方式
function urlToBase64(url, callback = null) {
    let image = new Image()

    image.setAttribute('crossOrigin', 'Anonymous')
    image.src = url + '?v=' + Math.random()

    image.onload = function () {
      let dataURL = imageToBase64(image)
      if (callback) {
        callback(dataURL)
      }
    }
}

// Promise方式
function urlToBase64Async(url) {
    return new Promise((resolve, reject) => {
      urlToBase64(url, (data) => {
        resolve(data)
      })
    })
}

(async () => {
    let image_url ='https://fuss10.elemecdn.com/a/3f/3302e58f9a181d2509f3dc0fa68b0jpeg.jpeg'

    const dataURL = await urlToBase64Async(image_url)
    console.log(dataURL)
    // data:image/png;base64,iVBORw0KGgoAAAANSU
})()

需要注意的是：

浏览器中有跨域问题的存在，所以浏览器端处理的图片地址，需要确保允许跨域