为什么Object.assign不复制URL对象的属性？

Question

On macOS 10.13.1 with Chrome 63 .

I'm using Object.assign with new URL() as the source object but it always gives an empty object? This seems like strange behavior. Here is my code:

 let url = new URL('http://www.yahoo.com');
 console.log(url);
 let data = Object.assign({}, url);
 console.log(data);

Why is data an empty object whereas url has the complete URL object as below:

{
 href: "http://www.yahoo.com/", 
 origin: "http://www.yahoo.com", 
 protocol: "http:", 
 username: "", 
 password: ""
 ...
}

I also tried:

let data = Object.assign({}, ...url);  

but it gives:

Uncaught TypeError: undefined is not a function

Answer 1

I suspect it's because the properties of URL are not enumerable . Notice when you do Object.keys(url) you also get a blank array? Both Object.assign and Object.keys work with enumerable properties.

Properties on your url object are not enumerable.

Answer 2

You can clone URL by simply:

 let url = new URL('http://www.yahoo.com');
 console.log(url);
 let data = new URL(url);
 console.log(data);

URL syntax:

url = new URL(url, [base])

You can still use an existing URL object for the base, which stringifies itself to the object's href attribute.