The has() method of the FormData interface returns whether a FormData object contains a certain key.

name

A string representing the name of the key you want to test for.

true if a key of FormData matches the specified name. Otherwise, false.

The following snippet shows the results of testing for the existence of username in a FormData object, before and after appending a username value to it with append():

formData.has("username"); // Returns false
formData.append("username", "Chris");
formData.has("username"); // Returns true

• Using FormData objects
• <form>