JSON Array is not updating

I am getting error on this line this.contactObj[row] = finall . My method is calling on cellchange event in table and I am using childobject data in table but when I am updating cell value in childObject I am getting error. Could anyone tell me is there any syntax error.

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

  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]))
    console.log('onl;lly  ' + this.contactObj[row])
    this.contactObj[row] = finall //getting error on this line
    //this.secondContact[row]= '[{"Id":"0035j00000MM49HAAT","FirstName":"Alok","LastName":"Thirdss","Phone":"44564754654","Email":"[email protected]"}]'
    // console.log(ts.replace(rs,queryString))
    //console.log(this.contactObj[row])
    //console.log(this.contactObj[row][field])

    // let temp2 = 
    // this.contactObj[row][field] = fieldValue; 
  }
}