let clients = [
{
id: "1",
weekDay: "Вт"
},
{id:"2",
weekDay: "Ср"
}
]
let newArr = clients.map((currentValue) => {
return currentValue.weekDay.toLowerCase()
})
let newArr = clients.map((currentValue) => {
currentValue.weekDay = currentValue.weekDay.toLowerCase();
return currentValue;
})