Cannot capture UpdateExpression like ++foo

[falsandtru]

class C {
  count_ = {};
  count() {
    return this.count_;
  }
}
var counter = new C();
assert(++counter.count_ === counter.count());

expected:

          assert(++counter.count_ === counter.count())
                 | |       |      |   |       |
                 | |       |      |   |       Object{}
                 | |       |      |   C{count_:#Object#}
                 | |       |      false
                 | |       Object{}
                 | C{count_:#Object#}
                 NaN

actual:

          assert(++counter.count_ === counter.count())
                 |                |   |       |
                 |                |   |       NaN
                 |                |   C{count_:NaN}
                 NaN              false

other:

          assert(+counter.count_ === counter.count())
                 ||       |      |   |       |
                 ||       |      |   |       Object{}
                 ||       |      |   C{count_:#Object#}
                 ||       |      false
                 ||       Object{}
                 |C{count_:#Object#}
                 NaN

[twada]

Hi, thank you for reporting.

Unfortunately, it's a limitation of power-assert. I cannot support UpdateExpression since it causes ReferenceError.

Let me explain. Given example.js,

'use strict';

function capt (val) {
    return val;
}

var counter = {
    count: 0
};

console.log(++counter.count); // => 1
console.log(capt(++capt(capt(counter).count)));  // => ReferenceError

results in ReferenceError when executed.

$ node example.js
1
~/src/github.com/power-assert-js/power-assert/example.js:12
console.log(capt(++capt(capt(counter).count)));
                   ^
ReferenceError: Invalid left-hand side expression in prefix operation
    at Object.<anonymous> (~/src/github.com/power-assert-js/power-assert/example.js:12:20)
    at Module._compile (module.js:460:26)
    at Object.Module._extensions..js (module.js:478:10)
    at Module.load (module.js:355:32)
    at Function.Module._load (module.js:310:12)
    at Function.Module.runMain (module.js:501:10)
    at startup (node.js:129:16)
    at node.js:814:3
$

capt function is what power-assert is doing internally, so when I tried to capture UpdateExpression like ++foo, it always result in ReferenceError.

Since test tool should not change behavior of SUT (System Under Test), I gave up supporting UpdateExpression.

Sorry for inconvenience. I'll write about this limitation to FAQ section in REAME. Thanks!

[falsandtru]

I understand, thanks!