Unexpected behavior in constant-folding-plugin when using dev=false

[chrisradek]

Do you want to request a feature or report a bug?
Bug

What is the current behavior?

When I declare (using var) a variable inside of a block, then check its 'truthy'-ness in a conditional outside that block, some code paths are removed.

Simplified example:

function foo(Bar) {
  if (typeof Bar != 'undefined') {
    var doesExist = true;
  }

  if (doesExist) {
    return Bar;
  } else {
    console.log('Bar does not exist');
  }
}

The constant-folding-plugin transforms this into:

function foo(Bar) {
  if (typeof Bar != 'undefined') {
    var doesExist = true;
  }

  {
    return Bar;
  }
}

If I move the doesExist declaration to be outside of the if statement, then the plugin outputs what I'd expect.

Updated input:

function foo(Bar) {
  var doesExist;
  if (typeof Bar != 'undefined') {
     doesExist = true;
  }

  if (doesExist) {
    return Bar;
  } else {
    console.log('Bar does not exist');
  }
}

Transformed output:

function foo(Bar) {
  var doesExist;

  if (typeof Bar != 'undefined') {
    doesExist = true;
  }

  if (doesExist) {
    return Bar;
  } else {
    console.log('Bar does not exist');
  }
}

If the current behavior is a bug, please provide the steps to reproduce and a minimal repository on GitHub that we can yarn install and yarn test.

What is the expected behavior?
Variables declared with var should be treated initially as undefined within a function scope, even when initialization occurs on the same line. Maybe this would only have to be true when a variable is declared and initialized within a function's inner-block. Given the above input, I would expect the output to look something like:

function foo(Bar) {
  if (typeof Bar != 'undefined') {
    var doesExist = true;
  }

  if (doesExist) {
    return Bar;
  } else {
    console.log('Bar does not exist');
  }
}

Please provide your exact Metro configuration and mention your Metro, node, yarn/npm version and operating system.

Using metro 0.38.3 with react-native 0.56.0. yarn version 1.7.0, npm version 6.2.0, node version 8.11.2.

[rafeca]

cc/ @mjesun I think you fixed this one, but the fix may not be available on metro 0.38.x

If that's the case, can you check if it's easy to cherry pick?

[mjesun]

That's a tricky one, and you're right, the removal of the if block in this case is not good. That's a bug in the path.evaluate function used by the constant folding plugin.

I've created an issue in Babel (babel/babel#8411), although it's not the first one I've already opened in regards to wrong evaluations (see babel/babel#8017).

[chrisradek]

Thanks for the update @mjesun