Blob

The Blob interface represents a blob, which is a file-like object of immutable, raw data; they can be read as text or binary data, or converted into a {{DOMxRef("ReadableStream")}} so its methods can be used for processing the data.

Blobs can represent data that isn't necessarily in a JavaScript-native format. The {{DOMxRef("File")}} interface is based on Blob, inheriting blob functionality and expanding it to support files on the user's system.

Using blobs

To construct a Blob from other non-blob objects and data, use the {{DOMxRef("Blob.Blob", "Blob()")}} constructor. To create a blob that contains a subset of another blob's data, use the {{DOMxRef("Blob.slice()", "slice()")}} method. To obtain a Blob object for a file on the user's file system, see the {{DOMxRef("File")}} documentation.

The APIs accepting Blob objects are also listed in the {{DOMxRef("File")}} documentation.

Constructor

{{DOMxRef("Blob.Blob", "Blob()")}}
- : Returns a newly created Blob object which contains a concatenation of all of the data in the array passed into the constructor.

Instance properties

{{DOMxRef("Blob.size")}} {{ReadOnlyInline}}
- : The size, in bytes, of the data contained in the Blob object.
{{DOMxRef("Blob.type")}} {{ReadOnlyInline}}
- : A string indicating the MIME type of the data contained in the Blob. If the type is unknown, this string is empty.

Instance methods

{{DOMxRef("Blob.arrayBuffer()")}}
- : Returns a promise that resolves with an {{jsxref("ArrayBuffer")}} containing the entire contents of the Blob as binary data.
{{DOMxRef("Blob.bytes()")}}
- : Returns a promise that resolves with a {{jsxref("Uint8Array")}} containing the contents of the Blob.
{{DOMxRef("Blob.slice()")}}
- : Returns a new Blob object containing the data in the specified range of bytes of the blob on which it's called.
{{DOMxRef("Blob.stream()")}}
- : Returns a {{DOMxRef("ReadableStream")}} that can be used to read the contents of the Blob.
{{DOMxRef("Blob.text()")}}
- : Returns a promise that resolves with a string containing the entire contents of the Blob interpreted as UTF-8 text.

Examples

Creating a blob

The {{DOMxRef("Blob.Blob", "Blob()")}} constructor can create blobs from other objects. For example, to construct a blob from a JSON string:

const obj = { hello: "world" };
const blob = new Blob([JSON.stringify(obj, null, 2)], {
  type: "application/json",
});

Creating a URL representing the contents of a typed array

The following example creates a JavaScript typed array and creates a new Blob containing the typed array's data. It then calls {{DOMxRef("URL/createObjectURL_static", "URL.createObjectURL()")}} to convert the blob into a {{glossary("URL")}}.

html

<p>
  This example creates a typed array containing the ASCII codes for the space
  character through the letter Z, then converts it to an object URL. A link to
  open that object URL is created. Click the link to see the decoded object URL.
</p>

The main piece of this code for example purposes is the typedArrayToURL() function, which creates a Blob from the given typed array and returns an object URL for it. Having converted the data into an object URL, it can be used in a number of ways, including as the value of the {{HTMLElement("img")}} element's src attribute (assuming the data contains an image, of course).

function showViewLiveResultButton() {
  if (window.self !== window.top) {
    // Ensure that if our document is in a frame, we get the user
    // to first open it in its own tab or window. Otherwise, this
    // example won't work.
    const p = document.querySelector("p");
    p.textContent = "";
    const button = document.createElement("button");
    button.textContent = "View live result of the example code above";
    p.append(button);
    button.addEventListener("click", () => window.open(location.href));
    return true;
  }
  return false;
}

if (!showViewLiveResultButton()) {
  function typedArrayToURL(typedArray, mimeType) {
    return URL.createObjectURL(
      new Blob([typedArray.buffer], { type: mimeType }),
    );
  }
  const bytes = new Uint8Array(59);

  for (let i = 0; i < 59; i++) {
    bytes[i] = 32 + i;
  }

  const url = typedArrayToURL(bytes, "text/plain");
  const link = document.createElement("a");

  link.href = url;
  link.innerText = "Open the array URL";
  document.body.appendChild(link);
}

{{EmbedLiveSample('url-from-array', , , , , , , 'allow-popups')}}

Extracting data from a blob

One way to read content from a Blob is to use a {{DOMxRef("FileReader")}}. The following code reads the content of a Blob as a typed array:

const reader = new FileReader();
reader.addEventListener("loadend", () => {
  // reader.result contains the contents of blob as a typed array
});
reader.readAsArrayBuffer(blob);

Another way to read content from a Blob is to use a {{domxref("Response")}}. The following code reads the content of a Blob as text:

const text = await new Response(blob).text();

Or by using {{DOMxRef("Blob.text()")}}:

const text = await blob.text();

By using other methods of FileReader, it is possible to read the contents of a Blob as a string or a data URL.