const mark = { firstName: `Mark`, lastName: `Miller`, fullName: this.firstName + this.lastName, weight: 78, height: 1.69 }; console.log(mark.fullName); // the result in console: NaNconst 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);