Merge two objects without override

With ES2015 now being supported in all modern browsers, the native Object.assign can be used to extend objects

Object.assign({}, _default, values)

Object.assign

Note that default is a reserved keyword, and can't be used as a variable name

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The original answer, written in 2013 :

Since this is tagged with jQuery, you could use $.extend for a simple cross-browser solution

var temp = {};
$.extend(true, temp, _default, values);
values = temp;

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For those who don't use jQuery, here comes a vanilla-js solution.

Solution:

function extend (target) {
    for(var i=1; i<arguments.length; ++i) {
        var from = arguments[i];
        if(typeof from !== 'object') continue;
        for(var j in from) {
            if(from.hasOwnProperty(j)) {
                target[j] = typeof from[j]==='object'
                ? extend({}, target[j], from[j])
                : from[j];
            }
        }
    }
    return target;
}

Compressed (with Closure Compiler):

Only 199 characters!

var extend=function e(c){for(var d=1;d<arguments.length;++d){var a=arguments[d];if("object"===typeof a)for(var b in a)a.hasOwnProperty(b)&&(c[b]="object"===typeof a[b]?e({},c[b],a[b]):a[b])}return c}

How to use:

extend(target, obj1, obj2); // returns target

If you only want to merge, use

var merged = extend({}, obj1, obj2);

Features:

• It doesn't look at objects' prototype.
• Ignores non-objects.
• It is recursive in order to merge properties which are objects.
• Objects referenced in target's properties, if extended, are replaced by new ones, and the original ones are not modified.
• In case of same property names, the merged value will be the merging of the objects after the last (in the order of arguments) non-object value. Or, if the last isn't an object, itself.

Examples:

extend({}, {a:1}, {a:2});            // {a:2}
extend({}, {a:1}, {b:2});            // {a:1, b:2}
extend({}, {a: {b:1}}, {a: {b:2}});  // {a: {b:2}}
extend({}, {a: {b:1}}, {a: {c:2}});  // {a: {b:2, c:2}}
extend({}, {a: {a:1}}, {a: {b:2}}, {a: 'whatever non object'});
    // {a: "whatever non object"}
extend({}, {a: {a:1}}, {a: {b:2}}, {a: 'whatever non object'}, {a: {c:3}},{a: {d:4}});
    // {a: {c:3, d:4}}

Warning:

Be aware that if browser is not clever enough, it could be trapped in an infinite loop:

var obj1={},
    obj2={};
obj1.me=obj1;
obj2.me=obj2;
extend({},obj1,obj2);

If the browser is clever enough, it can throw an error, or return {me: undefined}, or whatever.

Note that this warning also applies if you use jQuery's $.extend.

Also if you are happy with ES6:

Object.assign({}, default, values)