const mark = {
    firstName: `Mark`,
    lastName: `Miller`,
    fullName: function(){
                           return this.firstName + this.lastName;
                       },
    weight: 78,
     height: 1.69
};
console.log(mark.fullName());

Alternate:

const mark = ({
    firstName: `Mark`,
    lastName: `Miller`,
    init: function(){
                this.fullName = this.firstName + this.lastName;
            },
    weight: 78,
    height: 1.69
}).init();
console.log(mark.fullName);