Merging objects (associative arrays)

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

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 };

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