jQuery.extend with deep=true causes unpredicted side effects

[maciej-pawlisz]

There is a problem with deep extending - nested objects get overritten, which might be very confusing (example at: https://jsfiddle.net/4gpju44c/):

var nested={x:1}
var obj={n:nested}
$.extend(true,obj,{n:{x:2,y:2}}) 
console.log(nested)
{x:2, y:2, } 

It is even worse, when nested will be a prototype - we can get very unexpected errors (I experienced it when using jquery.rest and I changed verbs)

In jQuery.extend function there is a comment, which suggests that this situation should be avoided :

         // Never move original objects, clone them
        target[ name ] = jQuery.extend( deep, clone, copy );

but it is not working as the comment suggests. Maybe it should be replaced with:

        target[ name ] = jQuery.extend( deep,{}, clone, copy );

[dmethvin]

This is documented:

On a deep extend, Object and Array are extended, but object wrappers on primitive types such as String, Boolean, and Number are not. Deep-extending a cyclical data structure will result in an error.

The jQuery.extend is definitely quirky by today's standards, but the behavior is potentially depended upon by millions of web sites and jQuery itself so changing it would cause a lot of trouble. If you require other behavior, take a look at Object.assign() in ES6 or use lodash.