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javascriptsyntax
Peter Aron Zentai
Peter Aron Zentai

Reputation: 11740

JavaScript syntax madness: pointer to eval function is not the eval function?

The following needs to be in function scope, since the strange behavior does not occur is interactive console mode.

Following function returns 5 as expected

(function() { var x = 5; return eval("x"); })()

A simple transparent(ish) change:

(function() { var x = 5; var j = eval; return j("x"); })()

yields an error:

ReferenceError: x is not defined

Is this some kind of strange security measure?

Upvotes: 2

Views: 135

Answers (1)

James Allardice
James Allardice

Reputation: 166001

Your second example is an indirect call to eval. Indirect calls to eval are evaluated in the global scope, where x is not visible:

var x = 10;

// This will return 10
(function() { 
    var x = 5; 
    var j = eval; 
    return j("x");
})();

// This will return 5
(function() { 
    var x = 5; 
    return eval("x");
})();

From the spec:

 1. ...if the eval code is not being evaluated by a direct call to the eval function then

     a. Initialize the execution context as if it was a global execution context...

Upvotes: 6

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