import { LightningElement, api, track, wire } from 'lwc';
import { ShowToastEvent } from 'lightning/platformShowToastEvent';
import contactDetails from '@salesforce/apex/GetContact.getContacts';
import addContactRow from '@salesforce/apex/GetContact.addContactRow';
import deleteContactRow from '@salesforce/apex/GetContact.deleteContactRow';
const columns = [
    {
        label: 'Delete',
        type: 'button-icon',
        typeAttributes:
        {
            iconName: 'utility:delete',
            name: 'Delete',
            variant: 'brand',
        }
    },
    {
        label: 'FirstName',
        fieldName: 'FirstName',
        editable: true,
    },
    {
        label: 'LastName',
        fieldName: 'LastName',
        editable: true,
    },
    {
        label: 'Email',
        fieldName: 'Email',
        type: 'email',
        editable: true,
    },
    {
        label: 'Phone',
        fieldName: 'Phone',
        type: 'Phone',
        editable: true,
    },
];

export default class EOperation extends LightningElement {
    data = [];
    columns = columns;
    step = '1';
    @api recordId;
    boolvalue = true;
    emaildata = '';
    subject = '';
    body = '';
    hasError = false;
    error;
    contactObj;
    fldsItemValues = [];
    temp = [];
    secondContact
    @wire(contactDetails, {
        accId: '$recordId'
    }) wiredContacts({ error, data }) {
        if (data) {
            console.log('Temp Data is  ' + data)
            console.log('Stringify data ' + JSON.stringify(data))
            this.contactObj = data;
            this.secondContact = data;
            this.error = undefined;
        } else if (error) {
            console.log('error is ' + error)
            this.error = error;
            this.contactObj = undefined;
        }
    }
handleSave(event) {
        console.log('draft valus ID  ' + JSON.stringify(event.detail.draftValues));
        console.log('  type are ' + typeof (event.detail.draftValues) + '   IS Array ' + Array.isArray(event.detail.draftValues));
        this.temp.push(event.detail.draftValues);
       
        console.log('Temp 0 ' + JSON.stringify(this.temp['id']))
       
        let row = JSON.stringify(event.detail.draftValues).split(',')[1];
        row = row.substring(row.indexOf('-') + 1, row.indexOf('}') - 1)
        console.log('row is  ' + row)
       
        let field = JSON.stringify(event.detail.draftValues).split(',')[0] + '}';
        console.log('ffffis  ' + field)
       
        let fieldValue = field.substring(field.indexOf(':') + 2, field.indexOf('}') - 1);
        console.log('fieldvalue is ' + fieldValue)
        let queryString = '' + field.substring(field.indexOf('{') + 1, field.indexOf('}')) + '';
        console.log('queryString  ' + typeof (queryString))
       
        field = field.substring(field.indexOf('"') + 1, field.indexOf(':') - 1);
        console.log('field is sss ' + field)
       
        let val = this.contactObj[row][field]
        let rs = `"${field}":"${val}"`
       
        console.log('rs is ' + typeof (rs))
        console.log('   IS Array ' + Array.isArray(this.contactObj))
        //let ts1 = this.contactObj.splice(row,1)
        console.log(JSON.stringify(this.contactObj[row]).replace(rs, queryString))
        console.log('tss is  ')
       
        let finall = JSON.parse(JSON.stringify(this.secondContact[row]).replace(rs, queryString));
        //  this.contactObj[row] =JSON.parse(JSON.stringify(this.contactObj[row]).replace(rs,queryString))
       
        console.log(typeof (finall) + '  >>colomn are ' + typeof (this.secondContact[row]))
        this.contactObj[row]=finall;
        //this.secondContact[row]= '[{"Id":"0035j00000MM49HAAT","FirstName":"Alok","LastName":"Thirdss","Phone":"44564754654","Email":"bcd@gmail.com"}]'
        // console.log(ts.replace(rs,queryString))
        //console.log(this.contactObj[row])
        //console.log(this.contactObj[row][field])
        // let temp2 =
        // this.contactObj[row][field] = fieldValue;
    }
}

handelsave is called on oncellchange event in datatable. My Question is why i am getting error on this line 'this.contactObj[row]=finall;'. 
Actually I have created a datatable with inline edit & I am updating the childobject each time user update any cell.
Here's my datatable. I didn't add full code in JS I have mentioned my Line where I am getting my error. 

<template>
    <div style="height: 300px;">
        <lightning-card title="Contact Details">
            <lightning-button-icon icon-name="utility:adduser" variant="brand" alternative-text="Add user"
                class="slds-m-left_xx-small " slot="actions" title="AddUser" onclick={addUser}>
            </lightning-button-icon>
            <div>
            <lightning-datatable key-field="id" data={contactObj} suppress-bottom-bar hide-checkbox-column="true"
                onrowaction={handleRowActions} oncellchange={handleSave} draft-values={fldsItemValues}  columns={columns}>
            </lightning-datatable>
        </div>
            <p slot="footer">
                <lightning-button-icon icon-name="utility:save" variant="brand" alternative-text="Save"
                    class="slds-m-left_xx-small slds-float_right" title="Save" onclick={saveRecord}>

                </lightning-button-icon>
            </p>
        </lightning-card>


    </div>
</template>