obj.key3 = "abc";

obj['key3'] = "abc";

Object.assign(obj, {key3:'abc'});

Object.defineProperty(obj, 'key3', {
    value: "abc",       // undefined by default
    enumerable: true,      // false by default
    configurable: true,    // false by default
    writable: true         // false by default
  });

let ob = {...obj, key3:'abc'};

Object.assign({key3:'abc'}, obj);

let ob= JSON.parse(JSON.stringify(obj))
ob.key3 = 'abc';

let ob= Object.fromEntries(Object.entries(obj));
ob.key3 = 'abc';

Object.fromEntries(Object.entries(obj).concat([['key3','abc']]));