Does the comma operator influence the execution context in Javascript?

var a = 1;
var b = {
  a : 2,
  c : function () {
    console.log(this.a);
  }
};

b.c(); // logs 2
(b.c)(); // logs 2
(0, b.c)(); // logs 1

The first is understandable, for "this" is pointed to Object "b". But why does the second one log the same result? I thought "this" should be pointed to the global execution context. And the third one, it seems that the comma operator influences the execution context.

Answers:

Answer

You really have a nice corner case there! My take on it:

  • the first is straightforward. Just a standard call. The '.' operator lets you call the function setting b as the execution context.
  • the second is exactly the same thing: the parens are entirely optional and the interpreter is treating the expression inside it as a bound function call. Actually I didn't expect this: I thought the interpreter would be going to reset this to the global object, but actually it's keeping it linked. Probably just so "casual" language users do not freak out.
  • the third one is more standard (at least for those who live in JavaScript land): as soon as your function is passed on in an expression (in this case by the , operator) the this value is lost. This is because b.c is a Property Reference (deep rabbit hole details in the specification, here, courtesy of T.J.Crowder). So, you are actually passing around the function itself, no more bound to the declaring object. So when you call it this is going to be passed in as the global object.

See it this way: (object.function)() gets simplyfied into object.function(), because the enclosing parens are completely optional; (0, object.function)() is parsed as (expression yielding a function)() which is going to lose the object binding to this, because function is already unbound.

Really nice example!

Answer

Refer to Indirect eval call, which gives more details about it.

 (     0        ,          b.c   )        (  )
     |____|   |_____|    |_____|
     Literal  Operator   Identifier

     |_________________________|
     Expression

  |______________________________|
  PrimaryExpression

  |______________________________|        |________|
  MemberExpression                        Arguments

  |________________________________________________|

  CallExpression

We can use the comma operator to fashion an indirect call to b.c which will force it to execute in the global context, the value of a is 1 in the global context.

Also the result of (b.c = b.c)() is 1

> (b.c = b.c)()
  1

Speaking in terms of ECMAScript, this is because both — comma operator (in (0, b.c) example) and = operator (in (b.c = b.c) example) perform GetValue on its operands.

Other indirect call formats as below

> (b.c, b.c)()
  1 
> (1? b.c: 0)()
  1
> (__ = b.c)()
  1

Tags

Recent Questions

Top Questions

Home Tags Terms of Service Privacy Policy DMCA Contact Us

©2020 All rights reserved.