uglify-js@2.8.4 compresses the snippet

function foo() {
  'use strict';
  var myVar, myVar2;
  myVar = myVar2 = 42;
  return myVar;
}

console.log(foo());

to this:

uglifyjs --compress --mangle -- index.js 
WARN: Dropping unused variable myVar [index.js:3,5]
WARN: Dropping unused variable myVar2 [index.js:3,12]

function foo(){"use strict";return myVar2=42}console.log(foo());

Note that the variable declarations are dropped completely and myVar2 in the assignment seems to be wrongly considered to be a global variable. Due to strict mode, the minified version actually produces a ReferenceError now.

A similar case has also been reported in issue #1502, where myVar2 however was indeed a global variable.