带布尔值的Object.assign

Question

Whats the difference between

Object.assign({},obj1,obj2);

and

Object.assign(true,obj1,obj2);

I know what obj.assign does but whats the use of the latter one ? it returns a boolean type with obj1 and obj2 to merged into it.

PS: It was an interview question so would like to know whats the use case of this.

Answer 1

As outlined above, primitives get "boxed", so the following:

 Object.assign(true, obj1, obj2)

is merely the same as:

 const bool = Object.assign(new Boolean(true), obj1, obj2)

now boolean objects are just regular objects, that return a boolean when valueOf() is called on them. That means you get some funny behaviour:

 bool === true // false
 bool == true // true
 +bool === 1 // true

You cannot even use that Boolean object inside conditions as all objects are truthy:

  if(Object.assign(false,{a: 1 }))
   alert("works");

So actually there is no difference between a regular object and a Boolean object, except that the latter adds some confusion and serves no purpose whatsoever.

Answer 2

Besides boolean is supposed to be primitive type it has object-type wrapper that is... surprise Boolean.

In case of

Object.assign(true,{a: 1}, {b: 2})

there is such boxing(term from Java/C# worlds I really like) happens. And then Object.assign works over this object-type wrapper as it should. That's why result looks in console like

Boolean {true, a: 1, b: 2}

Boolean's object like every other objects can be extended with custom keys/values. Output looks confusing but if you expand this in console you will see there is no actually "value of true without any key" - so it's just tricky output.

BTW exactly the same happens to strings when you tries to call some method on primitive value like

'12345'.split(/./)

It is converted into object with String() constructor and then call String's method.

And yes, I believe it should be never used in real life code.

Here is spec says :

1. Let to be ToObject(target).

So explicit converting primitive-to-object is very first step.