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FormData: append() method

The append() method of the FormData interface appends a new value onto an existing key inside a FormData object, or adds the key if it does not already exist.

The difference between set() and append() is that if the specified key already exists, set() will overwrite all existing values with the new one, whereas append() will append the new value onto the end of the existing set of values.

Note: This method is available in Web Workers.

Syntax

js

append(name, value)
append(name, value, filename)

Parameters

name

The name of the field whose data is contained in value.

value

The field's value. This can be a string or Blob (including subclasses such as File). If none of these are specified the value is converted to a string.

filename Optional

The filename reported to the server (a string), when a Blob or File is passed as the second parameter. The default filename for Blob objects is "blob". The default filename for File objects is the file's filename.

Note: If you specify a Blob as the data to append to the FormData object, the filename that will be reported to the server in the "Content-Disposition" header used to vary from browser to browser.

Return value

None (undefined).

Examples

js

formData.append("username", "Chris");

When the value is a Blob (or a File), you can specify its name with the filename parameter:

js

formData.append("userpic", myFileInput.files[0], "chris.jpg");

As with regular form data, you can append multiple values with the same name:

js

formData.append("userpic", myFileInput.files[0], "chris1.jpg");
formData.append("userpic", myFileInput.files[1], "chris2.jpg");

If the value is not a string or a Blob, append() will convert it to a string automatically:

js

formData.append("name", true);
formData.append("name", 72);
formData.getAll("name"); // ["true", "72"]

Specifications

Browser compatibility

Desktop Mobile
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet
append 5 12
4Before Firefox 7, specifying a Blob as the data to append to the object, the filename reported in the Content-Disposition HTTP header was an empty string, resulting in errors on some servers. Starting with Firefox 7, the filename blob is sent.
10With the "Include local directory pass when uploading files to a server" option enabled, IE will change the filename inside the Blob on the fly. To have direct control of the sent filename, the developer should send the filename as the third parameter value, i.e. formData.append(name, value, filename).
12 5
3XHR in Android 4.0 sends empty content for FormData with blob.
18
4Before Firefox 7, specifying a Blob as the data to append to the object, the filename reported in the Content-Disposition HTTP header was an empty string, resulting in errors on some servers. Starting with Firefox 7, the filename blob is sent.
12 5 1.0
filename_parameter 16 12 22 10 ≤15 6 4.4 18 22 ≤14 6 1.0

See also

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https://developer.mozilla.org/en-US/docs/Web/API/FormData/append