How can I merge properties of two JavaScript objects dynamically?

I need to be able to merge two (very simple) JavaScript objects at runtime. For example I’d like to:

var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog' }

obj1.merge(obj2);

//obj1 now has three properties: food, car, and animal

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.

Answer

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-proposal-object-rest-spread plugin for it to work (This plugin is included in @babel/preset-env, in ES2018).

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;
}