is it a bad practice modifing an object passed as an argument inside a method?

Question

is it a bad practice modifing an object passed as an argument inside a method?

what is the difference between doSomething1 and doSomething2? the both does the same but in a different way. which are appropriate and why is that?

class Foo {
  constructor () {
    this.bar = { aaa: 123, bbb: 456 }
    this.doSomething1(this.bar)
    this.doSomething2()
  }

  doSomething1(arg) { arg.aaa = 10 },
  doSomething2() { 
    this.bar.bbb = 0
  }

}

the code itself is not practical but just a sample code.

Answer 1

That's the difference between a function and a method. A method, which is a function defined on an object, knows all the properties of the object and is allowed to modified them (also known as mutate or change the state). doSomething2 is a method of Foo class. Hence it should have the right or privilege to mutate any of the properties of an instance of Foo.

doSomething1 on the other hand is a function masquerading as a method. It has no right to modify the argument it receives and so shouldn't. If it really must, then create a new copy with the updated property instead.