angular break sub array values to main object. subarray size is dynamic

myArray = [{'ID' : 1,
        'addDetail' : ['test 1', 'test 2']}]

expected array

myArray = [{'ID' : 1, 'line1' : 'test 1', 'line2' : 'test 2'
        'addDetail' : ['test 1', 'test 2']}]

i used bellow login

this.partiesdatalist = this.partiesdatalist.map(item => {
const addDetail = item.addDetail;
addDetail?.forEach((detail, index) => {
  item[`line${index + 1}`] = detail;
});
return item;
});

but it showing error

Element implicitly has an ‘any’ type because expression of type ‘line${number}‘ can’t be used to index type

in the line of

item[`line${index + 1}`] = detail;