[FLEX-34984] for-each loops cross-compile incorrectly

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: Apache FlexJS 0.5.0
• Fix Version/s: Apache FalconJX 0.6.0
• Component/s: Falcon, FlexJS
• Assignee: Alex Harui
• Reporter: Andy Dufilie
• Created: 11/Dec/15 05:09
• Updated: 03/May/16 06:31
• Resolved: 11/Dec/15 20:13

Description

When you write a for-each loop, the iterable gets re-evaluated on every iteration.

Example code:

for each (var item:Object in Test.getItems()) {
	Test.doSomething(item);
}

Cross-compiled:

  for (var foreachiter0 in Test.getItems()) 
  {
  var item = Test.getItems()[foreachiter0];
  {
    Test.doSomething(item);
  }}

It should not call Test.getItems() on every loop iteration. Instead, it should generate a new local variable prior to entering the loop to store the result:

var foreachiter0_target = Test.getItems();
for (var foreachiter0 in foreachiter0_target)
{
    var item = foreachiter0_target[foreachiter0];
    Test.doSomething(item);
}

I can see problematic code in three places:

• JSGeneratingReducer.java: https://github.com/apache/flex-falcon/blob/develop/compiler.js/src/org/apache/flex/compiler/internal/as/codegen/JSGeneratingReducer.java#L3426
• ForEachEmitter.java: https://github.com/apache/flex-falcon/blob/develop/compiler.jx/src/org/apache/flex/compiler/internal/codegen/js/jx/ForEachEmitter.java
• JSVF2JSEmitter.java: https://github.com/apache/flex-falcon/blob/develop/compiler.jx/src/org/apache/flex/compiler/internal/codegen/js/vf2js/JSVF2JSEmitter.java#L1708

Another solution would be to use a for...of loop, Standard in ES6, though that won't work in Internet Explorer: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of