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FormData: append() method

files/en-us/web/api/formdata/append/index.md

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{{APIRef("XMLHttpRequest API")}} {{AvailableInWorkers}}

The append() method of the {{domxref("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 {{domxref("FormData.set", "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.

Syntax

js-nolint
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 {{domxref("Blob")}} (including subclasses such as {{domxref("File")}}). If none of these are specified the value is converted to a string.
  • filename {{optional_inline}}
    • : The filename reported to the server (a string), when a {{domxref("Blob")}} or {{domxref("File")}} is passed as the second parameter. The default filename for {{domxref("Blob")}} objects is "blob". The default filename for {{domxref("File")}} objects is the file's filename.

[!NOTE] If you specify a {{domxref("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 ({{jsxref("undefined")}}).

Examples

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

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

js
formData.append("user-pic", myFileInput.files[0], "chris.jpg");

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

js
formData.append("user-pic", myFileInput.files[0], "chris1.jpg");
formData.append("user-pic", 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

{{Specifications}}

Browser compatibility

{{Compat}}

See also