Duplicated names in var statements are considered as different variables

[RubenVerborgh]

1. Minimal code to reproduce the bug

Input Code

function f() {
  var a = 1;
  var b = a;
  var a = 2;
  console.log(a, b);
}
f();

or

function f() {
  var a = 1, b = a, a = 2;
  console.log(a, b);
}
f();

Live example on babeljs.io

Actual Output

function f(){var c=1,c=2;console.log(c,c)}f();

Expected Output

function f(){var c=1,d=c;c=2,console.log(c,d)}f();

Details

Note the incorrect constant propagation in the console.log statement: it has console.log(c,c), even though the two arguments in the original are different. This happens because the variable a is declared multiple times as var, yet is not recognized as the same variable.

In the correct interpretation a=2 is recognized as a modification to the same variable, so constant propagation is not allowed to happen. Changing the second var a to simply a makes the problem disappear.

Slightly more complex example:

function loop() {
  var end = 0;
  var start = end;
  while (end < 10) {
    console.log(start, end);
    var end = end + 1;
  }
}

2. Stack Trace if there is an Error thrown during minification

None.

3. Configuration - babel-minify configuration or babelrc

Default.

The problem is with the deadcode option (babel-plugin-minify-dead-code-elimination), which is switched on by default.

4. Whether you used it with other presets/plugins - like es2015 or env

None.

5. Environment: gulp, rollup, webpack, babel-cli, etc...?

All.