Nested Object Assign using Javascript

Question

    var obj1 = {
      a: "imgood",
      b: {
        a1: {
          a2: "i shouldnt be here",
          b2: "imgood"
        },
        b1: "imgood"
      }
    };
    
    var obj2 = {
      b: {
        a1: {
          a2: "imgood"
        }
      }
    };
    console.log(Object.assign(obj1,obj2));

I want the a2 to be replaced but without loosing other properties.

in Simplest, Shortest and Fastest way possible

Answer 1

Here is a solution with a for loop in a recursive function:

function recursiveAssign(a, b) {
    if (Object(b) !== b) return b;
    if (Object(a) !== a) a = {};
    for (let key in b) {
        a[key] = recursiveAssign(a[key], b[key]);
    }
    return a;
}

var obj1 = {
  a: "imgood",
  b: {
    a1: {
      a2: "i shouldnt be here",
      b2: "imgood"
    },
    b1: "imgood"
  }
};

var obj2 = {
  b: {
    a1: {
      a2: "imgood"
    }
  }
};

console.log(recursiveAssign(obj1, obj2));

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Or in a functional manner:

function recursiveAssign(a, b) {
    return Object(b) !== b ? b 
        : Object.keys(b).reduce ( (a, key) =>
            Object.assign(a, { [key]: recursiveAssign(a[key], b[key]) })
          , Object(a) === a ? a : {} );
}

var obj1 = {
  a: "imgood",
  b: {
    a1: {
      a2: "i shouldnt be here",
      b2: "imgood"
    },
    b1: "imgood"
  }
};

var obj2 = {
  b: {
    a1: {
      a2: "imgood"
    }
  }
};

console.log(recursiveAssign(obj1, obj2));

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