Apply changes from one object to another if they differ.

modify-what-differs.js

// Configs
var origConfig = {
  p: {
    a: 23,
    b: 92,
    c: {
      num: 1
    }
  },
  q: {
    x: 32,
    y: 29
  }
};

var diffConfig = {
  p: {
    c: {
      num: 3
    }
  },
  q: {
    x: 99
  },
  z: 23 // Unnecessary
};

// Holy Grail (Breadth First Traversal): Solution to our Problems!
function modifyWhatDiffers(original, diff) {
  var stack = [];

  if (!original || !diff) { 
    return original;
  }
  
  stack.push({
    o: original,
    d: diff
  });
  
  // Runs only for modifications in the diff object
  while (stack.length > 0) {
    let obj = stack.shift();

    for (var key in obj.d) {
      if (obj.o.hasOwnProperty(key) && obj.d.hasOwnProperty(key)) {
        if (typeof obj.o[key] === "object") {
          stack.push({
            o: obj.o[key],
            d: obj.d[key]
          });
        } else if (obj.o[key] !== obj.d[key]) {
          obj.o[key] = obj.d[key];
        }
      }
    }
  }

  return original;
}

// Pass in a Deep Copy of the Original, use any method you want!
let patchedConfig = modifyWhatDiffers(
  JSON.parse(JSON.stringify(origConfig)),
  diffConfig
);

console.log(JSON.stringify(origConfig));
console.log(JSON.stringify(diffConfig));
console.log(JSON.stringify(patchedConfig));

// {"p":{"a":23,"b":92,"c":{"num":1}},"q":{"x":32,"y":29}}
// {"p":{"c":{"num":3}},"q":{"x":99}},"z":23}
// {"p":{"a":23,"b":92,"c":{"num":3}},"q":{"x":99,"y":29}}