How to convert a File to a Blob in Javascript without FileReader?

Question

I'm trying to convert a File into a Blob without using FileReader. I can't use FileReader because the latest version of Typescript (which I must use) doesn't accept a File as the parameter to fileReader.readAsArrayBuffer().

Using FileReader I can do it as follows:

var reader = new FileReader();

reader.onloadend = function(e) {
  new Blob([ this.result ], { type: "image/jpeg" } );
};

reader.readAsArrayBuffer(file);

How can I achieve the same result without FileReader?

If I remember right, File is a subclass of Blob, so you shouldn't even need to use FileReader, it just already is a Blob. You could always cast to an any though if Typescript has issues with it. — loganfsmyth
– loganfsmyth, Commented May 2, 2017 at 17:12

Aluan Haddad · Accepted Answer · 2017-05-02 23:26:53Z

1

You can always augment the declaration in lib.d.ts with an overload accepting a File.

For example, create a file named globals.d.ts with the following content

interface FileReader {
    readAsArrayBuffer(file: File): void;
}

That said, if the declaration was changed, I would be wary of this behavior as some environments likely do not support it.

answered May 2, 2017 at 23:26

Aluan Haddad

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