Native way to merge objects in Javascript

Question

Javascript's Object doesn't have any native merge operation. If you have two objects, say

{a:1, b:2}
{c:3, d:4}

And want to get

{a:1, b:2, c:3, d:4}

As far as I know, you have to iterate through the objects. That is to say that you decide on either a merge left or merge right strategy and then you do something like (simplified)

for (key in object2) {
  object1[key] = object2[key];
}

This is fine. However, Javascript has the call and prototype feature. For instance, turning arguments into an Array can be done with

Array.prototype.slice.call(arguments)

This approach exploits existing native code, and so therefore is less susceptible to programmer folly and should run faster than a non-native implementation.

The question

Is there a trick to use this prototype/call pattern on perhaps the Attribute or Node traversal features of the DOM, or perhaps some of the generic String functions in order to do a native object merge?

The code would look something like this:

var merged = somethingrandom.obscuremethod.call(object1, object2)

And as a result, you'd get a native merge without a traversal.

A possible, sub-optimal solution

If you could use the constructor property of an Object and then coerce one object to have a constructor of another object and then run new over the composite object, you may get a merge for free. But I don't have a firm grasp of the full implications of the constructor feature in javascript to make this call.

Lemma

The same question holds true for Arrays. A common problem is to take, say 7 arrays of numbers, then try to find out the intersection of those arrays. That is to say, which numbers exist in all 7 arrays.

You could concat them together, then do a sort, and then do a traversal, surely. But it would be nice if there is a generic intersect tucked away somewhere that we can coerce an array to doing natively.

Any thoughts?

edit:

Getting half way there

For the array problem, you could do the following:

array.concat(a, b, c).sort().join(':') and then use some tricky RegExp capture and repeat patterns in order to traverse. RegExp implementations, if you don't know, run on a very simple stack-based virtual machine. When you initialize your regular expression that's really a program that gets compiled (RegExp.compile is a deprecated JS method). Then the native runs over the string in a blisteringly fast way. Perhaps you could exploit that for membership thresholds and get better performance...

It still doesn't go all the way though.

Answer 1

My answer to this will be disappointing, but still:

no

The reason for this is simple: Mr Resig's implementation of merge (or "extend" as it's called for objects) in jQuery is doing a loop, just like the one in your question. You can look at it here. And I dare say that if John Resig hasn't found a clever build-in way to do it, then the mere mortals of stackoverflow won't either :)

Answer 2

Using ES6 (ES2015) you can use Object.assign method:

var x = {a:1, b:2};
var y = {c:3, d:4};
var z = Object.assign({},x,y);

Using ES7 (ES2016, Chrome 60+ or Babel) you can use Object spread operator:

var x = {a:1, b:2};
var y = {c:3, d:4}; 
var z = {...x, ...y};

Answer 3

The million dollar question! I've tried doing this numerous ways, and the loop way described above always seemed the dirtiest. ES6's Object.setPrototypeOf() allows you to delegate a "property override" object to a "default properties" object, pretty much accomplishing what you're trying to do, but using Object.setPrototypeOf() has some serious implications, like disabling the browser's compiler optimizations for the whole script.

Also, in both the loop solution and the Object.setPrototypeOf() solution, you are left with a situation where the "property override" object can mutate the "default properties" object:

defaultObj = {
    a: [1, 2]
}
...
overrideObj = {
    b: 3
}
Object.setPrototypeOf(overrideObj, defaultObj);
console.log(overrideObj); // {a: [1, 2], b: 3}
// Great!
...
overrideObj.a.push(4);
console.log(defaultObj); // {a: [1, 2, 4]}
// Uh-oh.

You might think this is not a problem, but let's say you're using this object as configuration for a 3rd party lib. You are now handing the control of your default object and everything referenced in it to the 3rd party lib.

A better solution might be to use JSON.stringify and JSON.parse to copy and combine the objects. Here's a Gist with the example: https://gist.github.com/spikesagal/6f7822466887f19b9c65

HTH

Answer 4

Not that I know of, no. Also, you'll want to write your merge method like this:

function mergeInto(o1, o2) {
  if (o1 == null || o2 == null)
    return o1;

  for (var key in o2)
    if (o2.hasOwnProperty(key))
      o1[key] = o2[key];

  return o1;
}

Answer 5

You can do the following using native JS 1.7, without the need of a framework. See example on fiddle (example only intended for simple objects - not complex nested objects)

var obj1 = {a: "a", b: "b"};
var obj2 = {c: "c", d: "d"};

// The magic: ugly but works perfectly
var value = (JSON.stringify(obj1).concat(JSON.stringify(obj2))).replace("}{", ",");

document.getElementById("lbl1").setAttribute("value", value);

// back to object
var obj3 = JSON.parse(value);
document.getElementById("lbl2").setAttribute("value", obj3.a + " " + obj3.b + " " + obj3.c + " " + obj3.d);

Answer 6

You can combine the spread operator (...) and the Object.assign approach to get a good solution for the case where you have a bunch of objects in an array and want to merge them all into one super object.

const obj1 = {a:1, b:2}
const obj2 = {c:3, d:4}
const objArr = [obj1, obj2]
const mergedObj = Object.assign({}, ...objArr)

> { a: 1, b: 2, c: 3, d: 4 }

Answer 7

No native ways in ECMA-Script, use:

function merge(o1,o2) {
 if (typeof(o1)!=='object') o1={};
 if (typeof(o2)!=='object') o2={};
 for (var k in o2) {
 if (o1[k]!==undefined)
  alert ('Collision Error'); // TODO
 else
   o1[k]=o2[k];
 }
 return o1;
}

Answer 8

Below I've included a deep-merge function I wrote. It will not deep-merge Arrays, only Objects. It will take two objects, and return a third, new object.

var merge = function(o1, o2) {
    var o_new = {};
    for(p in o1) {
        if(o1[p]) {
            if(typeof o1[p] == 'object' && !(o1[p] instanceof Array) && o2.hasOwnProperty(p)) {
                o_new[p] = merge(o1[p], o2[p]);
            }
            else {
                o_new[p] = o1[p];
            }
        }
    }
    for(p in o2) {
        if(typeof o2[p] == 'object' && !(o2[p] instanceof Array) && o1.hasOwnProperty(p)) {
            o_new[p] = merge(o1[p], o2[p]);
        }
        else {
            o_new[p] = o2[p];
        }
    }
    return o_new;
}