Long story short, JSON.stringify takes optional second argument which can be a replacer function. It is supposed to be called for each encountered key-value during serialization. While replacer's second argument is already serialized (so you could just return it), just in case you are not satisfied with default serialization — the original value is still passed in together with the whole object as a value of JavaScript `this`. Sadly, there is no test in the whole ECMA test suite to catch this bug. This is quite a breaking change for code which already uses Qt-specific workarounds, so it would be better not to cherry-pick it as a hot-fix onto existing (released) branches. Sample use case: serialize dates as a number of seconds since epoch. function replacer(k, v) { if (this[k] instanceof Date) { return Math.floor(this[k].getTime() / 1000.0); } return v; } const str = JSON.stringify(obj, replacer); [ChangeLog][QML][Important Behavior Changes] Set correct `this` value in JSON.stringify replacer scope, so that the value for current key in current object is no longer pre-stringified, and can actually be used to implement custom object serialization logic. Fixes: QTBUG-95324 Pick-to: 6.5 Change-Id: I618a533e8eba7999d5416aca14f4971093a83f7a Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>