<p>I need to be able to merge two (very simple) JavaScript objects at runtime.  For example I'd like to:</p>
<pre><code>var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog' }

obj1.merge(obj2);

//obj1 now has three properties: food, car, and animal
</code></pre>
<p>Is there a built in way to do this?  I do not need recursion, and I do not need to merge functions, just methods on flat objects.</p>

ECMAScript 2018 Standard Method
You would use
object spread
:
```
```
let merged = {...obj1, ...obj2};
```
```
```
merged
```
is now the union of
```
obj1
```
and
```
obj2
```
. Properties in
```
obj2
```
will overwrite those in
```
obj1
```
.
```
```
/** There's no limit to the number of objects you can merge.
 *  Later properties overwrite earlier properties with the same name. */
const allRules = {...obj1, ...obj2, ...obj3};
```
```
Here is also the
MDN documentation
for this syntax. If you're using babel you'll need the
babel-plugin-transform-object-rest-spread
plugin for it to work.
ECMAScript 2015 (ES6) Standard Method
```
```
/* For the case in question, you would do: */
Object.assign(obj1, obj2);

/** There's no limit to the number of objects you can merge.
 *  All objects get merged into the first object. 
 *  Only the object in the first argument is mutated and returned.
 *  Later properties overwrite earlier properties with the same name. */
const allRules = Object.assign({}, obj1, obj2, obj3, etc);
```
```
(see
MDN JavaScript Reference
)
Method for ES5 and Earlier
```
```
for (var attrname in obj2) { obj1[attrname] = obj2[attrname]; }
```
```
Note that this will simply add all attributes of
```
obj2
```
to
```
obj1
```
which might not be what you want if you still want to use the unmodified
```
obj1
```
.
If you're using a framework that craps all over your prototypes then you have to get fancier with checks like
```
hasOwnProperty
```
, but that code will work for 99% of cases.
Example function:
```
```
/**
 * Overwrites obj1's values with obj2's and adds obj2's if non existent in obj1
 * @param obj1
 * @param obj2
 * @returns obj3 a new object based on obj1 and obj2
 */
function merge_options(obj1,obj2){
    var obj3 = {};
    for (var attrname in obj1) { obj3[attrname] = obj1[attrname]; }
    for (var attrname in obj2) { obj3[attrname] = obj2[attrname]; }
    return obj3;
}
```
```