netzaffin netzaffin - 1 month ago 9
Javascript Question

FormData.append("key", "value") is not working

Can you tell me whats wrong with this:

var formdata = new FormData();
formdata.append("key", "value");
console.log(formdata);


My output looks like this, I cant find my "key" - "value" pair

FormData
*__proto__: FormData
**append: function append() { [native code] }
***arguments: null
***caller: null
***length: 0
***name: "append"
***prototype: append
***__proto__: function Empty() {}
*constructor: function FormData() { [native code] }
**arguments: null
**caller: null
**length: 0
**name: "FormData"
**prototype: FormData
**toString: function toString() { [native code] }
*__proto__: Object
**__proto__: Object
**__defineGetter__: function __defineGetter__() { [native code] }
**__defineSetter__: function __defineSetter__() { [native code] }
**__lookupGetter__: function __lookupGetter__() { [native code] }
**__lookupSetter__: function __lookupSetter__() { [native code] }
**constructor: function Object() { [native code] }
**hasOwnProperty: function hasOwnProperty() { [native code] }
**isPrototypeOf: function isPrototypeOf() { [native code] }
**propertyIsEnumerable: function propertyIsEnumerable() { [native code] }
**toLocaleString: function toLocaleString() { [native code] }
**toString: function toString() { [native code] }
**valueOf: function valueOf() { [native code] }


I can't understand! Yesterday it worked so well, and today my head crashed the keyboard so many times! Firefox, Chrome, both the same :/

Answer

New in Chrome 50+ and Firefox 39+ (resp. 44+):

  • formdata.entries() (combine with Array.from() for debugability)
  • formdata.get(key)
  • and more very useful methods

Original answer:

What I usually do to 'debug' a FormData object, is just send it (anywhere!) and check the browser logs (eg. Chrome devtools' Network tab).

You don't need a/the same Ajax framework. You don't need any details. Just send it:

var xhr = new XMLHttpRequest;
xhr.open('POST', '/', true);
xhr.send(data);

Easy.