(1, eval)('this') vs eval('this

问题:

I start to read JavaScript Patterns, some codes confused me.

var global = (function () {
    return this || (1, eval)(\'this\');
}());

Here are my questions:

Q1:

(1, eval) === eval?

Why and how does it work?

Q2: Why not just

var global = (function () {
    return this || eval(\'this\');
}());

or

 var global = (function () {
    return this;
}());

回答1:

The difference between (1,eval) and plain old eval is that the former is a value and the latter is an lvalue. It would be more obvious if it were some other identifier:

var x;
x = 1;
(1, x) = 1; //  syntax error, of course!

That is (1,eval) is an expression that yields eval (just as say, (true && eval) or (0 ? 0 : eval) would), but it\'s not a reference to eval.

Why do you care?

Well, the Ecma spec considers a reference to eval to be a \"direct eval call\", but an expression that merely yields eval to be an indirect one -- and indirect eval calls are guaranteed to execute in global scope.

Things I still don\'t know:

1. Under what circumstance does a direct eval call not execute in global scope?
2. Under what circumstance can the this of a function at global scope not yield the global object?

Some more information can be gleaned here.

EDIT

Apparently, the answer to my first question is, \"almost always\". A direct eval executes from the current scope. Consider the following code:

var x = \'outer\';
(function() {
  var x = \'inner\';
  eval(\'console.log(\"direct call: \" + x)\'); 
  (1,eval)(\'console.log(\"indirect call: \" + x)\'); 
})();

Not surprisingly (heh-heh), this prints out:

direct call: inner
indirect call: outer

EDIT

After more experimentation, I\'m going to provisionally say that this cannot be set to null or undefined. It can be set to other falsy values (0, \'\', NaN, false), but only very deliberately.

I\'m going to say your source is suffering from a mild and reversible cranio-rectal inversion and might want to consider spending a week programming in Haskell.

回答2:

The fragment,

var global = (function () {  
    return this || (1, eval)(\'this\');  
}());  

will correctly evaluate to the global object even in strict mode. In non-strict mode the value of this is the global object but in strict mode it is undefined. The expression (1, eval)(\'this\') will always be the global object. The reason for this involves the rules around indirect verses direct eval. Direct calls to eval has the scope of the caller and the string this would evaluate to the value of this in the closure. Indirect evals evaluate in global scope as if they were executed inside a function in the global scope. Since that function is not itself a strict-mode function the global object is passed in as this and then the expression \'this\' evaluates to the global object. The expression (1, eval) is just a fancy way to force the eval to be indirect and return the global object.

A1: (1, eval)(\'this\') is not the same as eval(\'this\') because of the special rules around indirect verse direct calls to eval.

A2: The original works in strict mode, the modified versions do not.

回答3:

To Q1:

I think this is a good example of comma operator in JS. I like the explanation for comma operator in this article: http://javascriptweblog.wordpress.com/2011/04/04/the-javascript-comma-operator/

The comma operator evaluates both of its operands (from left to right) and returns the value of the second operand.

To Q2:

(1, eval)(\'this\') is considered as indirect eval call, which in ES5 does execute code globally. So the result will be the global the context.

See http://perfectionkills.com/global-eval-what-are-the-options/#evaling_in_global_scope

回答4:

Q1: Multiple consecutive javascript statements separated by a comma take the value of the last statement. So:

(1, eval) takes the value of the last one which is a function reference to the eval() function. It apparently does it this way to make the eval() call into an indirect eval call that will be evaluated in the global scope in ES5. Details explained here.

Q2: There must be some environment that doesn\'t define a global this, but does define eval(\'this\'). That\'s the only reason I can think of for that.