FULL PRODUCT VERSION :
java version "1.8.0_152-ea"
Java(TM) SE Runtime Environment (build 1.8.0_152-ea-b05)
Java HotSpot(TM) 64-Bit Server VM (build 25.152-b05, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
macOS 10.12.5

A DESCRIPTION OF THE PROBLEM :
In certain multithreading situations, some local variables are not being initialized properly. This seems to happen when a function declares multiple block-scoped variables that are captured by a closure, and when the function is invoked concurrently from multiple threads.

When this bug is triggered, the following code may throw a TypeError due to "x" being undefined:

const x = [1,2,3];
x.concat(4);

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the provided script several times, for example:

for i in {0..10}
do
  echo Attempt $i
  jjs --language=es6 crash.js
done

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The .js script should not generate any output, so the above 'for' loop should display the following:

Attempt 0
Attempt 1
Attempt 2
Attempt 3
Attempt 4
Attempt 5
Attempt 6
Attempt 7
Attempt 8
Attempt 9
Attempt 10
ACTUAL -
This test case reproduces the issue about 50% of the time.

ERROR MESSAGES/STACK TRACES THAT OCCUR :
Attempt 0
Attempt 1
Attempt 2
Attempt 3
Attempt 4
Attempt 5
TypeError: Cannot read property "crash" from undefined
TypeError: Cannot read property "crash" from undefined
Attempt 6
Attempt 7
TypeError: Cannot read property "crash" from undefined
TypeError: Cannot read property "crash" from undefined
Attempt 8
TypeError: Cannot read property "crash" from undefined
TypeError: Cannot read property "crash" from undefined
Attempt 9
TypeError: Cannot read property "crash" from undefined
TypeError: Cannot read property "crash" from undefined
Attempt 10
TypeError: Cannot read property "crash" from undefined
TypeError: Cannot read property "crash" from undefined

REPRODUCIBILITY :
This bug can be reproduced often.

---------- BEGIN SOURCE ----------
function f() {
   let a;
   const b = {};
   b.crash; // b is sometimes undefined

   function() {
       a, b;
   }
}

T = Java.extend(Java.type('java.lang.Thread'), {
    run: function() {
        for (let j = 0; j < 100; j++) {
            try {
                f();
            }
            catch (e) {
                print(e);
                break;
            }
        }
    }
});

new T().start();
new T().start();

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Either of the following will eliminate the issue, in this particular test case:

- Declare 'a' as 'var'
- Declare 'b' as 'let' or 'var'