I am assuming this is javascript (based on the question's tag). In which case the code you are trying to execute seems wrong.

Solution to Question

var list = [{firstName: "James", lastName: "M.", age: 58}, {firstName: "Rachel", lastName: "W.", age: 51}];

list.forEach((entry) => {
   entry.fullName = entry.firstName + " " + entry.lastName; 
});

console.log(list[0].fullName);

In this solution, I am simply looping through all the elements of the array and assigning a new property "fullName" with the values of the element's own property.

There are multiple ways to do it, this is just the most simplest way to clear your understanding.

To display the individual property

console.log(list[0].firstName);

Note, the array index accessor is after the array name list and not after the attribute/property name firstName in this case.

Also in your provided code there is no attribute firstName so do have some checks in place else it will use undefined value and might throw some error depending on your development environment.

Computed Attribute (getter accessor)

You should also think about how the array data will behave if the value of the attributes/properties changes.

For example if you change the firstName attribute/property then you need to remember to update the fullName attribute/property as well, else they will be out of sync.

The more efficient way to do this task is to have the attribute/property as a computed attribute/property (think functions or more accurately getters)

Sample Code

var list = [{
  firstName: "James",
  lastName: "M.",
  age: 58,
  get fullName() {
    return this.firstName + " " + this.lastName;
  },
}, {
  firstName: "Rachel",
  lastName: "W.",
  age: 51,
  get fullName() {
    return this.firstName + " " + this.lastName;
  },
}];


console.log(list[0].fullName);

You can learn more about getters as well as setters at JavaScript Accessors (Getters and Setters)

I see two bugs in your code.

1. The key for first name is 'name' in your list of objects and you've been trying the use 'firstName' while concatenating.

2. You have been trying to merge the properties of objects in the for loop, where you need to merge the values.

Here's the Solution:

var list = [{firstName: "James", lastName: "M.", age: 58}, {firstName: "Rachel", lastName: "W.", age: 51}];

list.forEach(function(itm){
    itm.username = itm.firstName + " " + itm.lastName; });

console.log(list)

Since the objects are in the list, you will need to provide an index to access the object and its values.

Examples:

console.log(list[0].username)

Or to get the list of usernames only, you can use:

userNames = list.map(function(obj) {
    return obj.username;
});

console.log(userNames)

Well, there's no property called list.firstName, I think you mean list.name.

The solution is to create a new object called list.username and combine list.name and list.lastName and since you are inside forEach function, list will be called itm. So, it will be itm.name and itm.lastName.

I have also added + " " + to create a space between the first name and the second name.

var list = [{name: "James", lastName: "M.", age: 58}, {name: "Rachel", lastName: "W.", age: 51}];

list.forEach(function(itm) {
    list.username = itm.name + " " + itm.lastName;

    console.log(list.username);

    /**
    James M.
    Rachel W.
    **/
});

As well as there's no property called list.firstName, you can't access list.firstName, try accessing another one as well.

var list = [{name: "James", lastName: "M.", age: 58}, {name: "Rachel", lastName: "W.", age: 51}];
    
console.log(list[0].name); // James