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in chrome 47 and nodejs v0.12

new Function('myArg','return "my function body";')

gives the following results :

function anonymous(myArg /**/) {
  return "my function body"
}

why is there ments /**/ in the function arguments ?

in chrome 47 and nodejs v0.12

new Function('myArg','return "my function body";')

gives the following results :

function anonymous(myArg /**/) {
  return "my function body"
}

why is there ments /**/ in the function arguments ?

• javascript
• node.js

Share Improve this question Follow asked Oct 20, 2015 at 15:07 fadomirefadomire 1,97511 gold badge1515 silver badges2323 bronze badges 13

• 5 They needed it to fix the arguments parser for ES6. Wait, I'll dig up the bug discussion – Bergi Commented Oct 20, 2015 at 15:11
• 3 Here's the short explanation (as ment) in the source of the V8 engine: "If the formal parameters include an unbalanced block ment, the function must be rejected. Since JavaScript does not allow nested ments we can include a trailing block ment to catch this." – Andreas Commented Oct 20, 2015 at 15:13
• 1 So, it's just a way of making 'myArg /*' not throw an error? or what. – Kevin B Commented Oct 20, 2015 at 15:17
• 1 @KevinB yes it seems to be the reason. i'm wondering why it don't throw an error in case you put an unblanced ment instead of allowing it and fixing it with this workaround – fadomire Commented Oct 20, 2015 at 15:25
• 2 Ah, I was actually refering to bugzilla.mozilla/show_bug.cgi?id=755821 - seems Firefox is going to do the same – Bergi Commented Oct 20, 2015 at 16:24

 |  Show 8 more ments

2 Answers 2

Sorted by: Reset to default 8

As seen in the following Chromium issue, this is a workaround to remedy an edge case involving unbalanced block ments. As described in the V8 source code:

function NewFunctionString(arguments, function_token) {
  var n = arguments.length;
  var p = '';
  if (n > 1) {
    p = ToString(arguments[0]);
    for (var i = 1; i < n - 1; i++) {
      p += ',' + ToString(arguments[i]);
    }
    // If the formal parameters string include ) - an illegal
    // character - it may make the bined function expression
    // pile. We avoid this problem by checking for this early on.
    if (%_CallFunction(p, ')', StringIndexOfJS) != -1) {
      throw MakeSyntaxError('paren_in_arg_string', []);
    }
    // If the formal parameters include an unbalanced block ment, the
    // function must be rejected. Since JavaScript does not allow nested
    // ments we can include a trailing block ment to catch this.
    p += '\n/' + '**/';
  }
  var body = (n > 0) ? ToString(arguments[n - 1]) : '';
  return '(' + function_token + '(' + p + ') {\n' + body + '\n})';
}

This was originally added to catch cases like the following and throw an error:

Function("/*", "*/){alert('bad');")

This should result in a syntax error, but before they added the additional /**/, it would be translated to:

function anonymous(/*) {
  */){alert('bad');
}

Which is equivalent to

function anonymous(/*...*/) {
  alert('bad');
}

And hence no syntax error. After the change, with the extra ment this now bees:

function anonymous(/*/**/) {
  */){alert('bad');
}

which correctly gives a syntax error:

Uncaught SyntaxError: Unexpected token *(…)

Source revision and original bug report.

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