Bug: Object literals in unused variables

[loilo]

Raised by squashing ami.js

Unused local variables are discarded, their values are kept. That leads to issues if the respective value is an object literal which is invalid as a standalone statement.

I think UglifyJS provides a good solution by recursively walking the object literal and extracting all possible side effects, discarding the object afterwards.

I'm not sure why bare variable access is considered a side effect, but I assume there are good reasons for that?

Input:

function wrapper () {
  var any_var = {
    prop_1: any_value,
    prop_2: any_fn()
  }
}

Output Butternut 0.4.6:

function wrapper(){{prop_1:any_value,prop_2:any_fn()}}

Output UglifyJS 3.0.8:

function wrapper(){any_value,any_fn()}

[loilo]

Sometimes the object literal cannot be omitted, e.g. when a method is called on it. In this case it should be made sure that it will be put in parantheses.

Input:

function wrapper () {
  var any_variable = {}.toString()
}

Output Butternut 0.4.6:

function wrapper(){{}.toString()}

Output UglifyJS 3.0.8:

function wrapper(){({}).toString()}