javascript 如何从 Blob 到 ArrayBuffer
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How to go from Blob to ArrayBuffer
提问by Jeanluca Scaljeri
I was studying Blobs, and I noticed that when you have an ArrayBuffer, you can easily convert this to a Blob as follows:
我正在研究 Blob，我注意到当你有一个 ArrayBuffer 时，你可以很容易地将它转换为一个 Blob，如下所示：
var dataView = new DataView(arrayBuffer);
var blob = new Blob([dataView], { type: mimeString });
The question I have now is, is it possible to go from a Blob to an ArrayBuffer?
我现在的问题是，是否可以从 Blob 转到 ArrayBuffer？
采纳答案by tripulse
The ResponseAPI consumes a (immutable) Blobfrom which the data can be retrieved in several ways. The OPonly asked for ArrayBuffer, and here's a demonstration of it.
所述ResponseAPI消耗（不可变）Blob从该数据可以以几种方式进行检索。在OP只问ArrayBuffer，这里是它的一个示范。
var blob = GetABlobSomehow();
// NOTE: you will need to wrap this up in a async block first.
/* Use the await keyword to wait for the Promise to resolve */
await new Response(blob).arrayBuffer(); //=> <ArrayBuffer>
alternatively you could use this:
或者你可以使用这个：
new Response(blob).arrayBuffer()
.then(/* <function> */);
Note:This APIisn't compatible with older (ancient) browsers so take a look to the Browser Compatibility Tableto be on the safe side ;)
注意：此API与较旧的（古老的）浏览器不兼容，因此请查看浏览器兼容性表以确保安全；)
回答by potatosalad
You can use FileReaderto read the Blobas an ArrayBuffer.
您可以使用FileReader将 阅读Blob为ArrayBuffer.
Here's a short example:
这是一个简短的例子：
var arrayBuffer;
var fileReader = new FileReader();
fileReader.onload = function(event) {
arrayBuffer = event.target.result;
};
fileReader.readAsArrayBuffer(blob);
Here's a longer example:
这是一个更长的例子：
// ArrayBuffer -> Blob
var uint8Array = new Uint8Array([1, 2, 3]);
var arrayBuffer = uint8Array.buffer;
var blob = new Blob([arrayBuffer]);
// Blob -> ArrayBuffer
var uint8ArrayNew = null;
var arrayBufferNew = null;
var fileReader = new FileReader();
fileReader.onload = function(event) {
arrayBufferNew = event.target.result;
uint8ArrayNew = new Uint8Array(arrayBufferNew);
// warn if read values are not the same as the original values
// arrayEqual from: http://stackoverflow.com/questions/3115982/how-to-check-javascript-array-equals
function arrayEqual(a, b) { return !(a<b || b<a); };
if (arrayBufferNew.byteLength !== arrayBuffer.byteLength) // should be 3
console.warn("ArrayBuffer byteLength does not match");
if (arrayEqual(uint8ArrayNew, uint8Array) !== true) // should be [1,2,3]
console.warn("Uint8Array does not match");
};
fileReader.readAsArrayBuffer(blob);
fileReader.result; // also accessible this way once the blob has been read
This was tested out in the console of Chrome 27—69, Firefox 20—60, and Safari 6—11.
这在 Chrome 27-69、Firefox 20-60 和 Safari 6-11 的控制台中进行了测试。
Here's also a live demonstration which you can play with: https://jsfiddle.net/potatosalad/FbaM6/
这里还有一个你可以玩的现场演示：https: //jsfiddle.net/potatosalad/FbaM6/
Update 2018-06-23:Thanks to Klaus Klein for the tip about event.target.resultversus this.result
2018-06-23 更新：感谢 Klaus Klein 提供关于event.target.resultvs的提示this.result
Reference:
参考：
回答by Klaus Klein
Just to complement Mr @potatosalad answer.
只是为了补充@potatosalad 先生的回答。
You don't actually need to access the function scopeto get the result on the onloadcallback, you can freely do the following on the eventparameter:
您实际上并不需要访问函数作用域来获取onload回调的结果，您可以自由地对event参数执行以下操作：
var arrayBuffer;
var fileReader = new FileReader();
fileReader.onload = function(event) {
arrayBuffer = event.target.result;
};
fileReader.readAsArrayBuffer(blob);
Why this is better? Because then we may use arrow function without losing the context
为什么这样更好？因为那样我们就可以在不丢失上下文的情况下使用箭头函数
var fileReader = new FileReader();
fileReader.onload = (event) => {
this.externalScopeVariable = event.target.result;
};
fileReader.readAsArrayBuffer(blob);
回答by Arlen Beiler
Or you can use the fetch API
或者你可以使用 fetch API
fetch(URL.createObjectURL(myBlob)).then(res => res.arrayBuffer())
I don't know what the performance difference is, and this will show up on your network tab in DevTools as well.
我不知道性能差异是什么，这也会显示在 DevTools 的网络选项卡上。
回答by Kaiido
There is now(Chrome 76+ & FF 69+) a Blob.prototype.arrayBuffer()method which will return a Promise resolving with an ArrayBuffer representing the Blob's data.
还有现在器（Chrome 76+＆FF 69+）一Blob.prototype.arrayBuffer（）将返回一个承诺与代表blob数据的ArrayBuffer解决方法。
(async () => {
const blob = new Blob(['hello']);
const buf = await blob.arrayBuffer();
console.log( buf.byteLength ); // 5
})();