最近在項目中有這么一個需求，就是上傳一個視頻文件，然后要獲取視頻文件的第一幀圖片，這個可以通過canvas獲取得到，得到的是一個dataURL,之后還要將這個圖片上傳到云，這個時候如何操作就不清楚了，于是乎，google一番，總結如下：

1. 將dataURL轉成Blob
2. 利用formData
3. 異步上傳

function b64toBlob(b64Data, contentType='', sliceSize=512) {
        const byteCharacters = atob(b64Data);
        const byteArrays = [];

        for (let offset = 0; offset < byteCharacters.length; offset += sliceSize) {
            const slice = byteCharacters.slice(offset, offset + sliceSize);

            const byteNumbers = new Array(slice.length);
            for (let i = 0; i < slice.length; i++) {
                byteNumbers[i] = slice.charCodeAt(i);
            }

            const byteArray = new Uint8Array(byteNumbers);

            byteArrays.push(byteArray);
        }

      const blob = new Blob(byteArrays, {type: contentType});
      return blob;
}

// dataURL to blob

// 假設一個dataURL
const ImageURL = "data:image/jpeg;base64,abcdefg

// Split the base64 string in data and contentType
const block = ImageURL.split(";");

// Get the content type of the image
const contentType = block[0].split(":")[1];// In this case "image/jpeg"

// get the real base64 content of the file
const realData = block[1].split(",")[1];// In this case "R0lGODlhPQBEAPeoAJosM...."

// Convert it to a blob to upload
var blob = b64toBlob(realData, contentType);

// new a formData

const formData = new FormData();
formData.append('blob', blob);

// upload

$.ajax({
    url:url,
    data: formData
    type:"POST",
    contentType:false,
    processData:false,
    error:function(err){
    },
    success:function(data){
    },
});