Javascript named function expression, reference error

Question

Javascript named function expression, reference error

I am getting "Reference Error" very randomly (about once in 200 attempts ) with the following code.

var securityPrototype = {   
    init: function(){ /* ... */ },
    encryptionKey: function x() {
        var i = x.identifier; 
        return getKey(i);
    }
}

securityPrototype.encryptionKey.identifier = Date.now();

function Security(){}

Security.prototype = securityPrototype;
Security.constructor = Security;

function getKey(){ /* ... */ }

var gate = new Security()
gate.encryptionKey();  // Randomly throws : ReferenceError: x is not defined

This code segment lives inside other code but no "eval" is being used , neither the 'with' operator.

I am trying to figure out if due to any condition it is possible to get this error here.

Browser that reproduces this: Chrome on Mac and Windows. IE and Safari work fine.

node.js
google-chrome
javascript
javascript-objects

Answer 1

That's because a bug in the implementation of named function expressions, that exists in some versions of some browsers.

In those browsers, two separate function objects are created when you use a named function expression. The property gate.encryptionKey is a reference to one function object, and the name x is a reference to a different function object. They both contain the same code, but they are different instances of the Function class.

When you assign a value to gate.encryptionKey.identifier, that property is only available in the function object that gate.encryptionKey references. The function object that x references doesn't have that property.

Simple example of the behaviour in those browsers:

var f = function g(){};
f === g; // false

f.expando = 'foo';
g.expando; // undefined

This example (example #3) was taken from the page Named function expressions demystified, where you can read more about named function expressions, and the implementation bugs.