Merging objects (associative arrays) [duplicate]

with jquery you can call $.extend

var obj1 = {a: 1, b: 2};
var obj2 = {a: 4, c: 110};

var obj3 = $.extend(obj1, obj2); 

obj1 == obj3 == {a: 4, b: 2, c: 110} // Pseudo JS

(assoc. arrays are objects in js)

look here: http://api.jquery.com/jQuery.extend/

edit: Like rymo suggested, it's better to do it this way:

obj3 = $.extend({}, obj1, obj2); 
obj3 == {a: 4, b: 2, c: 110}

As here obj1 (and obj2) remain unchanged.

edit2: In 2018 the way to do it is via Object.assign:

var obj3 = Object.assign({}, obj1, obj2); 
obj3 === {a: 4, b: 2, c: 110} // Pseudo JS

If working with ES6 this can be achieved with the Spread Operator:

const obj3 = { ...obj1, ...obj2 };

Now in 2016 I would say the best/standard way is Object.assign()
Pure Javascript. No jQuery is needed.

obj1 = {a: 1, b: 2};
obj2 = {a: 4, c: 110};
obj3 = Object.assign({},obj1, obj2);  // Object {a: 4, b: 2, c: 110}

More information, examples and polyfill here:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign

This is how Prototype does it:

Object.extend = function(destination, source) {
    for (var property in source) {
        if (source.hasOwnProperty(property)) {
            destination[property] = source[property];
        }
    }
    return destination;
};

called as, for example:

var arr1　= { robert: "bobby", john: "jack" };
var arr2 = { elizabeth: "liz", jennifer: "jen" };

var shortnames = Object.extend(arr1,arr2);

EDIT: added hasOwnProperty() check as correctly pointed out by bucabay in comments

Keep it simple...

function mergeArray(array1,array2) {
  for(item in array1) {
    array2[item] = array1[item];
  }
  return array2;
}

Underscore also has an extend method:

Copy all of the properties in the source objects over to the destination object. It's in-order, so the last source will override properties of the same name in previous arguments.

_.extend(destination, *sources) 

_.extend({name : 'moe'}, {age : 50});
=> {name : 'moe', age : 50}