JS Object Update + React Reducer

I’m struggling to update all “new” attributes too false. Below is an example object.

const msgdata = {
"1511207758": {
    "userid": "1000015977",
    "text": "hello",
    "new": false,
    "subjecttitle": null,
    "subjectcontent": null,
    "datetime": "2017-11-20T19:55:58.000Z"
},
"1511277428": {
    "userid": "1000000000",
    "text": "hey hi",
    "new": false,
    "subjecttitle": null,
    "subjectcontent": null,
    "datetime": "2017-11-21T15:17:08.000Z"
},
"1640341426": {
    "userid": "1000000000",
    "text": "hellow how are you?",
    "new": false,
    "subjecttitle": null,
    "subjectcontent": null,
    "datetime": "2021-12-24T10:23:45.706Z"
},
"1640342296": {
    "userid": "1000000000",
    "text": "asdfasdf",
    "new": true,
    "subjecttitle": null,
    "subjectcontent": null,
    "datetime": "2021-12-24T10:38:16.089Z"
},
"1640342382": {
    "userid": "1000000000",
    "text": "fxvfv",
    "new": true,
    "subjecttitle": null,
    "subjectcontent": null,
    "datetime": "2021-12-24T10:39:41.910Z"
}

}

when I try the following I mess up the object structure changes to an array but I need to maintain the object/key structure – just change the new value to false.

    let newMessages = {}
    if (payload.length > 0) {
        payload.map((item) => {
            if (messagesData && messagesData[item.userid]) {
                newMessages = Object.keys(messagesData[item.userid].messages).map((key) => ({ ...newMessages[item.userid].messages[key], new: false }))
            }
            return true
        })

        console.dir('newMessages')
        console.dir(newMessages)

the return object is a standard array – map does that… eg: 0,1,2 keys…

How can I maintain the object structure and just change the new attr.

thx