Object.assign with boolean

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.

There is no sane use of the second one. I would hope never to see that in a codebase, ever. — CertainPerformance
– CertainPerformance, Commented Oct 6, 2018 at 7:09
@CertainPerformance I agree, i was wondering the same but since they asked in the interview i thought it might be important. — Hith
– Hith, Commented Oct 6, 2018 at 7:10
It's not important, it's just silly trivia I think. Yes, Boolean objects can have properties assigned to them, but they really shouldn't. — CertainPerformance
– CertainPerformance, Commented Oct 6, 2018 at 7:11
Object.assign converts the first argument to an object if it isn't already (ecma-international.org/ecma-262/9.0/#sec-object.assign). Boolean objects don't have any use. — Felix Kling
– Felix Kling, Commented Oct 6, 2018 at 7:11

Jonas Wilms · Accepted Answer · 2018-10-06 07:28:50Z

2

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.

answered Oct 6, 2018 at 7:28

Jonas Wilms

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1 Comment

skyboyer Over a year ago

just side note: .valueOf() returns primitive value back. So it's possible to unbox it for using it in if().

skyboyer · Accepted Answer · 2018-10-06 07:21:40Z

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:

Let to be ToObject(target).

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

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