SyntaxError: Unexpected token ) for Arrow function in node 6

[kevinprotoss]

• Version: v6.2.0
• Platform: Linux client3 4.0.0n-fuse deps: update openssl to 1.0.1j #1 SMP PREEMPT Wed Apr 15 10:05:27 CEST 2015 x86_64 Intel(R) Core(TM) i7-2860QM CPU @ 2.50GHz GenuineIntel GNU/Linux
• Subsystem:

The code is run with harmony already. It works in node 5, but no hope in node 6. What's the reason of that.

/Invend/int_modules/restore-cache/lib/userCache.js:44
  cb = cb || () => {};
              ^
SyntaxError: Unexpected token )
    at Object.exports.runInThisContext (vm.js:53:16)
    at Module._compile (module.js:513:28)
    at Object.Module._extensions..js (module.js:550:10)
    at Module.load (module.js:458:32)
    at tryModuleLoad (module.js:417:12)
    at Function.Module._load (module.js:409:3)
    at Module.require (module.js:468:17)
    at require (internal/module.js:20:19)
    at Object.<anonymous> (/Invend/int_modules/restore-cache/lib/index.js:29:17)
    at Module._compile (module.js:541:32)
    at Object.Module._extensions..js (module.js:550:10)
    at Module.load (module.js:458:32)
    at tryModuleLoad (module.js:417:12)
    at Function.Module._load (module.js:409:3)
    at Module.require (module.js:468:17)
    at require (internal/module.js:20:19)
    at Object.<anonymous> (/Invend/int_modules/restore-application-provider/index.js:6:16)
    at Module._compile (module.js:541:32)
    at Object.Module._extensions..js (module.js:550:10)
    at Module.load (module.js:458:32)
    at tryModuleLoad (module.js:417:12)
    at Function.Module._load (module.js:409:3)
    at Module.require (module.js:468:17)
    at require (internal/module.js:20:19)
    at Object.<anonymous> (/Invend/egapiserver/server.js:11:24)
    at Module._compile (module.js:541:32)
    at Object.Module._extensions..js (module.js:550:10)
    at Module.load (module.js:458:32)

Meanwhile I found the following syntax works:

cb = () => {};

[vsemozhetbyt]

This also works:

cb = cb || (() => {});

Is || precede => in operator precedence?

[kevinprotoss]

@vsemozhetbyt I guess so

[vsemozhetbyt]

Firefox engine gives this error for the first case:
SyntaxError: invalid arrow-function arguments (parentheses around the arrow-function may help)

[kevinprotoss]

Does that mean the parentheses around the arrow function is always required?

[vsemozhetbyt]

It seems not always:

// No need for parentheses even for lower-precedence expression body
let square = x => x * x;

=> parses as if it were a low-precedence (assignment) operator joining a restricted comma expression (implicitly quoted) to a body.
This avoids precedence inversion when the body is an AssignmentExpression.
No preference for object literal over block (see [[strawman:block vs object literal]] to keep grammar simple and match expression-statement.

http://tc39wiki.calculist.org/es6/arrow-functions/

However, I don't completely understand the last remarks.

[kevinprotoss]

Yes, it's not required for such case. However, it looks like the parentheses is needed if together with || because of the operator precedence

[bnoordhuis]

Yes, || has higher precedence than =>. It makes sense when you consider that if it was the other way around, then you'd have to write x => x || 42 as x => (x || 42), which is awkward.

I'll close, not a bug.