Should i always put class property inside a constructor in js?

Is there any difference in JS between

class a{
    constructor(){
     this.something=“blah balh”
    }
}

And just

class a{
   something=“blah blah”
}

I know i need a constructor when i pass specific data for the new instance. But when the data is the same for all instances, is there any difference between these 2 ways? Should i use one of them over the other?