obj = [ {prop11: ‘value1’,prop21: ‘value2’}, {prop12: ‘value1’,prop22: ‘value2′}, {key3: {}} ]; obj.forEach(element => { Object.defineProperty(obj.key3,’prop1′,{value:”xx”,writable:true,enumerable:true,configurable:true}) }); console.log(obj.prop1) getting error – Uncaught TypeError: undefined is not a non-null object tried several ways of this and also Object.defineProperties() but couldn’t get working. javascript arrays object properties