[SalesForce] LWC data table JS issue: “cannot add property accountName, object is not extensible”

APEX CLASS

 public with sharing class datatabletest{
    @AuraEnabled(cacheable=true)
    public static List<Theme__c> getData(string userId) {
        return [SELECT Account__r.name, Account__c, Name, Risk_Level__c 
                FROM Theme__c 
                WHERE (Risk_Level__c != null AND Risk_Action__c = null)
                AND Account__r.OwnerId =: userId];
    }
  }

snippet of JS: you can see where my main issue it.

// retrieving the data using wire service
    @wire(getData, {userId: '$UserId'})
    themeData(result) {



         // get the account name
        if (result.data) { 


            let dataParse;
            let datas = result.data;
            for (let i = 0; i < datas.length; i++) {
                 dataParse = datas[i];
                if (dataParse.Account__r) dataParse.accountName = dataParse.Account__r.Name;

            }
            this.data = dataParse;

Html of the table, just if someone asks

          <template if:true={data}>

           <lightning-datatable data={data}
                                columns={columns}
                                key-field="id"
                                hide-checkbox-column="true"
                                onrowaction={handleRowActions}></lightning-datatable>
       </template>

   </div>

Now the reason i'm querying both the Id of the account and the account__r.name is because in my columns section where i define the columns i'm going to create a link to the record id but give it the label of the actual name….

also on an action i need the Id values because i'm going to use a lightning-record-edit-form to create a related object record and i pass the id values from the row…now all of that works but i just can't get the damn name of the account to display on the data table.

For those curious my edit form is at the bottom of this post.

unrelated: Also i noticed something weird.

    {
"Account__c":"0013000000bBCerAAG",
"Name":"Hard Wood",
"Risk_Level__c":"Medium",
"Id":"a4VQ0000000niplMAA",
    "Account__r":
    {
"Name":"Table Mountain",
"Id":"0013000000bBCerAAG"
    }
},

Even though i don't query the Theme__c Id it gives it to me anyways. Also when i query Account__r.Name it returns a Id value do i even need to query the Account__c field directly for the Id value?? It's just so frustrating, there has to be some easier way of accessing those account__r values when creating the columns of the data table.

Here's the edit form for those curious, value= {vars i defined in js} it automatically passes the id values to those lookup fields. if the edit form doesn't have a recordId defined then it just a create new based on the object in object-api-name

           <div if:true={isEditForm} class="slds-theme_default">
           <lightning-record-edit-form layout-type="Full"  object-api-name="Theme_Action__c" onsubmit={handleSubmit} onsuccess={handleSuccess}>
               <lightning-messages></lightning-messages>

               <lightning-input-field field-name="Account__c" value={Account_Id} ></lightning-input-field>
               <lightning-input-field field-name="heme__c" value={themeId}></lightning-input-field>
               <lightning-input-field field-name="Action__c"></lightning-input-field>
               <br/>

               <div style="text-align:center;">
                   <lightning-button class="slds-m-top_small"
                                     variant="brand"
                                     type="submit"
                                     name="update"
                                     label="Create Record"></lightning-button>
               </div>
           </lightning-record-edit-form><br/>
           <div></div>
       </div>

EDIT:

i changed up my JS

         // flatten out data
    if (result.data){ 
        console.log(' this data' + JSON.stringify(result.data));    

        let dataParse;
        for (let i = 0; i <  result.data.length; i++) {             
            dataParse = Object.assign({},  result.data[i]);
            if (dataParse.Account__r) dataParse.accountName = dataParse.Account__r.Name;


        }

        this.data = dataParse;

Now when i console log dataParse outside of the loop i get this

 {"Account__c":"0013000000bBCerAAG","Name":"WOF Pink Diamond 3R1L2C $5","Risk_Level__c":"High","Id":"a4VQ0000000nmqmMAA","Account__r":
{"Name":"hard wood","Id":"0013000000bBCerAAG"},"accountName":"Table Mountain Casino"}

and that's it. just one record.

when console.log the initial result.data

    [{"Account__c":"0013000000bBCerAAG","Name":"FORTUNE GONG with DRAGON  ASC CURVE","Risk_Level__c":"Medium","Id":"a4VQ0000000niplMAA",
"Account__r":{"Name":"hard wood","Id":"0013000000bBCerAAG"}},
{"Account__c":"0013000000bBCerAAG","Name":"WOF CL EXOTIC FAR EAST ASC","Risk_Level__c":"Medium","Id":"a4VQ0000000njLyMAI",
"Account__r":{"Name":"hard wood","Id":"0013000000bBCerAAG"}},
{"Account__c":"0013000000bBCerAAG","Name":"CD+ ASC - JIN LONG 888 3R","Risk_Level__c":"High","Id":"a4VQ0000000njhqMAA",
"Account__r":{"Name":"hard wood","Id":"0013000000bBCerAAG"}},
{"Account__c":"0013000000bBCerAAG","Name":"REEF OF RICHES 3D","Risk_Level__c":"High","Id":"a4VQ0000000njsrMAA",
"Account__r":{"Name":"hard wood","Id":"0013000000bBCerAAG"}},
{"Account__c":"0013000000bBCerAAG","Name":"TEMPLE OF TREASURE TPL GOLD ASC","Risk_Level__c":"Medium","Id":"a4VQ0000000nkMjMAI",
"Account__r":{"Name":"hard wood","Id":"0013000000bBCerAAG"}},
{"Account__c":"0013000000bBCerAAG","Name":"ZUMA 3D","Risk_Level__c":"Medium","Id":"a4VQ0000000nkRGMAY",
"Account__r":{"Name":"hard wood","Id":"0013000000bBCerAAG"}},
{"Account__c":"0013000000bBCerAAG","Name":"ANYTHING BUT SIX 3D STPR : AXXIS_3D","Risk_Level__c":"Medium","Id":"a4VQ0000000nkxzMAA",
"Account__r":{"Name":"hard wood","Id":"0013000000bBCerAAG"}},
{"Account__c":"0013000000bBCerAAG","Name":"WOF Pink Diamond 3R1L2C $5","Risk_Level__c":"High","Id":"a4VQ0000000nmqmMAA",
"Account__r":{"Name":"hard wood","Id":"0013000000bBCerAAG"}}]

I think it has to do with the account name being the same