在Javascript中将base64转换为文件

目标是将base64字符串转换为可发送的jpg文件，我不能使用html输入类型文件，但我必须以相同的格式提供服务。我有点失去了文件生成。(我在一个客户端移动应用程序)。

这是我所拥有的:

 file = "data:image/jpg;base64,#{imageData}"

imageData是base64字符串

有办法把它转换成一个有效的文件吗?

免责声明:产生无效的结果(接近，但无效)

我上周早些时候做了相反的事情——也就是说，将图像加载为二进制数据(以绕过从localhost运行file的要求)。

在其中，I:

加载文件
base64转换成
为base64字符串
将构造的字符串设置为img元素的src

这工作得很好。在看到你的问题后，我试图简单地逆转这个过程。然而，我在某个地方没有成功。数据被正确地从图像中提取出来，然后在某个地方(我认为在对atob的调用中取消编码)，数据被弄乱了。

保存的文件大小出乎意料，在"%PNG"之前添加了一个字符，并且在文件中间有一些丢失的数据。老实说，在这一点上我很困惑。

总之，这是我试过的代码:

1。读取文件并将数据填充到元素

的代码

// fileVar is an object as returned by <input type='file'>
// imgElem is an <img> element - (doesn't need to be added to the DOM)
function loadImgFromFile(fileVar, imgElem)
{
    var fileReader = new FileReader();
    fileReader.onload = onFileLoaded;
    fileReader.readAsBinaryString(fileVar);
    function onFileLoaded(fileLoadedEvent)
    {
        var result,data;
        data = fileLoadedEvent.target.result;
        result = "data:";
        result += fileVar.type;
        result += ";base64,";
        result += btoa(data);
        imgElem.src = result;
    }
}

2。尝试从图像/画布抓取数据，并使用程序员提供的文件名强制下载。

<!doctype html>
<html>
<head>
<script>
function byId(e){return document.getElementById(e)}
function newEl(tag){return document.createElement(tag)}
window.addEventListener('load', onPageLoaded, false);
function onPageLoaded(evt)
{
    var imgElem = byId('srcImg');
    imgElem.onload = function(){saveImgAsFile( byId('srcImg'), "myImage.png" );};
        // simple result of canvas.toDataURL() called on a 5x5 pixel image of a '+'
    imgElem.src = "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHklEQVQIW2NkQID/QCYjiAsmoABFEMRBAThVYmgHAAhoBQWHhfyYAAAAAElFTkSuQmCC"; 
    // use the below line instead of the one above, if you wish to assign an actual image file, rather than the result of call to canvas.toDataURL()
    // the base64 string allows me to keep it all in the one file, also, to run if opened via a double-click, rather than having to run from localhost
//  imgElem.src = "img/1x1.png";
}
function saveImgAsFile(imgElem, fileName)
{
    // get a base64 encoded string from an image element
    var srcElem = imgElem;
    var dstCanvas = newEl('canvas');
    dstCanvas.width = srcElem.naturalWidth;
    dstCanvas.height = srcElem.naturalHeight;
    var ctx = dstCanvas.getContext('2d');
    ctx.drawImage(srcElem,0,0);
    var imgSrcStr = dstCanvas.toDataURL();
    // extract the image type
    var colonPos = imgSrcStr.indexOf(":");
    var semiColonPos = imgSrcStr.indexOf(";");
    var imgType = imgSrcStr.slice(colonPos+1, semiColonPos);
    console.log("image type: " + imgType);
    // extract the image data
    var commaPos = imgSrcStr.indexOf(',');
    var base64ImgString = imgSrcStr.slice(commaPos + 1);
    console.log("Data: " + base64ImgString);
    // holds the data that is actually written to disk for this image
    //** I think the error occurs during this step **//
    var unencodedImage = atob(base64ImgString);
    var imgFileAsBlob = new Blob( [unencodedImage], {type: imgType} );
    var fileNameToUse = fileName;
    var downloadLink = newEl('a');
    downloadLink.download = fileNameToUse;
    downloadLink.innerHTML = "Download File";
    if (window.webkitURL != null)
    {
        // Chrome allows the link to be clicked
        // without actually adding it to the DOM.
        downloadLink.href = window.webkitURL.createObjectURL(imgFileAsBlob);
    }
    else
    {
        // Firefox requires the link to be added to the DOM
        // before it can be clicked.
        downloadLink.href = window.URL.createObjectURL(imgFileAsBlob);
        downloadLink.onclick = destroyClickedElement;
        downloadLink.style.display = "none";
        document.body.appendChild(downloadLink);
    }
    downloadLink.click();
}
/*
function saveTextAsFile()
{
    var textToWrite = "This is just some random content";
    var textFileAsBlob = new Blob([textToWrite], {type:'text/plain'})
    var fileNameToSaveAs = "myFile.txt";
    var downloadLink = document.createElement("a");
    downloadLink.download = fileNameToSaveAs;
    downloadLink.innerHTML = "Download File";
    if (window.webkitURL != null)
    {
        // Chrome allows the link to be clicked
        // without actually adding it to the DOM.
        downloadLink.href = window.webkitURL.createObjectURL(textFileAsBlob);
    }
    else
    {
        // Firefox requires the link to be added to the DOM
        // before it can be clicked.
        downloadLink.href = window.URL.createObjectURL(textFileAsBlob);
        downloadLink.onclick = destroyClickedElement;
        downloadLink.style.display = "none";
        document.body.appendChild(downloadLink);
    }
    downloadLink.click();
}
*/
function destroyClickedElement(event)
{
    document.body.removeChild(event.target);
}
</script>
</head>
<body>
    <img id='srcImg'/>
</body>
</html>

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