图片Image转换为base64编码的方法

1、FileReader

通过XMLHttpRequest请求图片Blob数据格式，然后利用FileReader转换为dataURL

function toDataURL(url, callback) {
  var xhr = new XMLHttpRequest();
  xhr.onload = function() {
    var reader = new FileReader();
    reader.onloadend = function() {
      callback(reader.result);
    }
    reader.readAsDataURL(xhr.response);
  };
  xhr.open('GET', url);
  xhr.responseType = 'blob';
  xhr.send();
}
 
toDataURL('https://www.gravatar.com/avatar/d50c83cc0c6523b4d3f6085295c953e0', function(dataUrl) {
  console.log('RESULT:', dataUrl)
})

请求可以利用 WHATWG fetchJs来实现

const toDataURL = url => fetch(url)
  .then(response => response.blob())
  .then(blob => new Promise((resolve, reject) => {
    const reader = new FileReader()
    reader.onloadend = () => resolve(reader.result)
    reader.onerror = reject
    reader.readAsDataURL(blob)
  }))
 
toDataURL('https://www.gravatar.com/avatar/d50c83cc0c6523b4d3f6085295c953e0').then(dataUrl => {
    console.log('RESULT:', dataUrl)
})

优缺点：

• 某些浏览器缺乏支持
• 更好的文件压缩
• 适用其他文件类型

查看支持：

• http://caniuse.com/#feat=filereader
• http://caniuse.com/#feat=fetch
• CORS Image

2、Canvas

新建图片，在画布上实现，然后将canvas对象转变为一个dataURL

function toDataURL(src, callback, outputFormat) {
  var img = new Image();
  img.crossOrigin = 'Anonymous'; // 允许image请求的图片应用到canvas，就像他们在同一个域
  img.onload = function() {
    var canvas = document.createElement('CANVAS');
    var ctx = canvas.getContext('2d');
    var dataURL;
    canvas.height = this.naturalHeight;
    canvas.width = this.naturalWidth;
    ctx.drawImage(this, 0, 0);
    dataURL = canvas.toDataURL(outputFormat);
    callback(dataURL);
  };
  img.src = src;
  if (img.complete || img.complete === undefined) { // 确保对缓存的图片也触发img.onload事件
    img.src = "data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///ywAAAAAAQABAAACAUwAOw==";
    img.src = src;
  }
}
 
toDataURL(
  'https://www.gravatar.com/avatar/d50c83cc0c6523b4d3f6085295c953e0',
  function(dataUrl) {
    console.log('RESULT:', dataUrl)
  }
)

支持的inputFormat：

• image/png, image/jpeg, image/jpg, image/gif, image/bmp, image/tiff, image/x-icon, image/svg+xml, image/webp, image/xxx

支持的outputFormat：

• image/png, image/jpeg, image/webp(chrome)

查看支持：

• http://caniuse.com/#feat=canvas
• IE10 (IE10 just works with same origin images)

3、从当前文件系统读取，input读取文件后，利用FileReader进行转换

function encodeImageFileAsURL(element) {
  var file = element.files[0];
  var reader = new FileReader();
  reader.onloadend = function() {
    console.log('RESULT', reader.result)
  }
  reader.readAsDataURL(file);
}

<input type="file" onchange="encodeImageFileAsURL(this)" />