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From: Cezary Tomczyk <cezary.tomczyk@gmail.com>
Newsgroups: comp.lang.javascript
Subject: Re: Object.defineProperties
Date: Sun, 7 May 2017 10:06:43 +0200
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Xref: csiph.com comp.lang.javascript:33248

On 07/05/2017 00:29, Stefan Weiss wrote:
> Cezary Tomczyk wrote:
>>     function isTypeOf(obj, typeToCheck) {
>>         var type;
>>
>>         if (arguments.length < 2) {
>>             throw new Error('[isTypeOf] requires two arguments');
>>         }
>>
>>         type = typeToCheck.toLowerCase();
>>
>>         if (type === 'null' && obj === null) {
>>             return 'null';
>>         }
>>
>>         if (type === 'array' && Array.isArray(obj)) {
>>             return 'array';
>>         }
>>
>>         return typeof obj === type;
>>     }
>
> It's shorter, but I still don't see the advantage of this:
>
>   isTypeOf(value, "null")
>   isTypeOf(value, "string")
>   isTypeOf(value, "array")
>
> over this:
>
>   value === null
>   typeof value == "string"
>   Array.isArray(value)
>
> There may be a use for a function that can examine values using a
> dynamically provided type - for example in a debugger, something like "list
> all object properties, sorted by type, and separating nulls and arrays".
>
> But I'm wondering if you actually need it. In another post, you wrote:
>
> | One of the reason is because I use it in other function that extends
> | deeply one object with another. I can't use Object.assign as the
> | changes on extended object must not mutate original object.
>
> Could you elaborate on that? Object.assign() only changes the first of its
> arguments (the target); the rest stays intact. There are other caveats with
> Object.assign(), but the source object should not be changed (unless its
> getters are doing something strange).

Here is the example case:

var o = { a: [1] },
     c = {};

Object.assign(c, o);

console.log(o.a[0]);

1

console.log(c.a[0]);

1

c.a[0] = 2;

console.log(o.a[0]);

2

but should be 1.

So, Object.assign does shallow copy, not deep copy. My goal , by having 
"extend" function in that case, is to do deep copy.

If I do extend({}, o); then new object will have his own data and 
changes made on new object must not reflect on the source object.

So far I've done:

     function extend(target, source) {
         var output = target;

         function processProperty(key) {
             if (isTypeOf(source[key], 'object')) {
                 output[key] = {};
                 extend(output[key], source[key]);
             } else if (isTypeOf(source[key], 'array')) {
                 output[key] = source[key].slice(0);
             } else {
                 output[key] = source[key];
             }
         }

         Object.keys(source).forEach(processProperty);

         return output;
     }

Unit test passes when I run them in the browser.

                 it('should extend new object with specified object 
(deep copy)', function () {
                     var o = {
                             id: 1,
                             arr: [
                                 {
                                     example: 'example'
                                 }
                             ],
                             nodes: [
                                 document.createElement('span')
                             ],
                             str: 'str'
                         },
                         obj = object.extend({}, o);

                     obj.id = 2;
                     obj.arr[0].example = 'example2';
                     obj.nodes[0] = document.createElement('div');
                     obj.str = 'str2';

                     expect(o.id).toBe(1);
                     expect(o.arr[0].example).toBe('example');
                     expect(o.nodes[0].nodeName.toLowerCase()).toBe('span');
                     expect(o.str).toBe('str');
                 });


However, when I run it in PhantomJS 2.1.1 (Mac OS X) then it throws 
exception:

"TypeError: Requested keys of a value that is not an object. in 
object.js (line 9)"

While I understand that "source" is passed as a non-object then I can't 
find out what's causing it. At least debugging PhantomJS is a bit hard.

-- 
Cezary Tomczyk
http://www.ctomczyk.pl/