lodash merge actually does what I exactly need.

https://blog.mariusschulz.com/2015/03/30/combining-settings-objects-with-lodash-assign-or-merge

// Defined within your component
var defaultSettings = {
    strictMode: true,
    formatting: {
        finalNewline: true,
        quotes: "double"
    }
};

// Provided by the developer using your component
var userSettings = {
    formatting: {
        quotes: "single"
    }
};

var assignedSettings = _.assign({}, defaultSettings, userSettings);

// {
//     strictMode: true,
//     formatting: {
//         quotes: "single"
//     }
// }

var mergedSettings = _.merge({}, defaultSettings, userSettings);

// {
//     strictMode: true,
//     formatting: {
//         finalNewline: true,
//         quotes: "single"
//     }
// }

1) Keep only shared keys in the second object.

2) Then use the spread operator.

let obj1 ={a:'a1',b:'b1'};
let obj2 ={b:'b2',c:'c2'};

let obj2Shared = Object.keys(obj2).reduce((acc, currKey) => {
  if (obj1[currKey]) {
    acc[currKey] = obj2[currKey];
  }
  return acc;
}, {});

let combined = {...obj1, ...obj2Shared};

console.log(combined);