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Object.prototype.toString.call(form) returns "[object Object]" instead of "[object FormData]" #396
Description
This appears to be due to a change in the algorithm used for determining the toString of an Object. See step 16 of 19.1.3.6 Object.prototype.toString ( )
const form = new FormData();
assert(form.toString() == "[object FormData]", "toString() results in [object FormData]")
assert(Object.prototype.toString.call(form) == "[object FormData]", "Object.prototype.toString.call(form) results in [object FormData]")In version 2.3.2 of FormData, the second assertion will fail as Object.prototype.toString.call(form) returns [object Object]
The fix for this is to add support for the toStringTag symbol, like the following:
if (Symbol && Symbol.toStringTag) {
Object.defineProperty(FormData.prototype, Symbol.toStringTag, {
value: 'FormData',
writable: false,
enumerable: false,
configurable: true
});
}
Activity
henryozoebuka commentedon Feb 12, 2025
ljharb commentedon Feb 12, 2025
ThisIsMissEm commentedon Feb 12, 2025
henryozoebuka commentedon Feb 13, 2025
ljharb commentedon Feb 13, 2025
[Fix] set `Symbol.toStringTag` when available
[Fix] set `Symbol.toStringTag` when available
[Fix] set `Symbol.toStringTag` when available (#573)
[Fix] set `Symbol.toStringTag` when available (#573)
ljharb commentedon Feb 14, 2025
Released in v4.0.2 and backported to v3.0.3 and v2.5.3.
engPabloMartinez commentedon Nov 17, 2025
I can still see this same behaviour on 4.04
ljharb commentedon Nov 17, 2025
@engPabloMartinez on which engine?
form-data/lib/form_data.js
Line 491 in 16e0076
engPabloMartinez commentedon Nov 17, 2025
I am seeing this in a NextJS application on the server side, so NodeJS.
const formData = new FormData();
formData.append('file', fs.createReadStream(
/tmp/${fileName}));formData.append(
'type',
mimeType === 'image/jpg' ? 'image/jpeg' : mimeType
);
console.log('Prototype: ', Object.prototype.toString.call(formData));
console.log('toString: ', formData.toString());
And I get
Prototype: [object Object]
toString: [object FormData]
ljharb commentedon Nov 17, 2025
Thanks, I've reproduced it and will have a fix out shortly.
[Fix] set Symbol.toStringTag in the proper place
ljharb commentedon Nov 17, 2025
v4.0.5 is released.
engPabloMartinez commentedon Nov 17, 2025
Thanks, now it is working.