Function to add new key:value to an array of objects

The first problem is that your addKeyAndValue function doesn't return anything. Remove the redundant braces to make it return the results of the map function.

const addKeyAndValue = (arr, key, value) => arr.map(obj => ...);

The second problem is that the arrow function you've passed to map also doesn't return anything. You will end up with an array containing the same number of elements, but each element will be undefined. Make the arrow function return something:

arr.map(obj => {
    obj[key] = value;
    return obj;
});

Demo[^]

However, as you can see from the demo, although map returns a new array, it's only a shallow copy. Your new property is added to the elements in the source array as well, since they're pointing to the same objects.

If you don't want to modify the elements in the source array, you will need to copy them. The simplest option is probably to use the spread operator:

arr.map(obj => ({ ...obj, [key]: value}));

Demo[^]
Spread syntax (...) - JavaScript | MDN[^]

const names = [
{name: 'Elie'},
{name: 'Tim'},
{name: 'Matt'},
{name: 'Colt'}
]
const addKeyAndValue = (arr, key, value) => {
arr.forEach(object => {
object[key] = value
})
console.log(arr)
return arr
}
addKeyAndValue(names, 'title', 'instructor')