Internet Explorer: "console is not defined" Error

Question

I was using console.log() in some JavaScript I wrote and an error of: console is not defined was thrown in Internet Explorer (worked fine in other browsers).

I have replaced it with:

if (console) console.log("...");

If console is undefined, I would expect the condition to evaluate as false. Ergo, the statement console.log wouldn't be executed and shouldn't throw an error.

Instead, an error of: console is not defined at character 4 is thrown.

Is this a IE bug? Or is that "if" condition really illegal? It seems absurd because if if (console) is illegal, then if (console==undefined) should be illegal too.

How are you supposed to check for undefined variables?

Answer 1

Other answers gave you the root cause. However, there's a better solution than using if before any call to console.*

Add this (once) before including any of your scripts that use console:

//Ensures there will be no 'console is undefined' errors window.console = window.console || (function(){     var c = {}; c.log = c.warn = c.debug = c.info = c.error = c.time = c.dir = c.profile = c.clear = c.exception = c.trace = c.assert = function(s){};     return c; })(); 

This will create a 'pseudo' console only if it doesn't exist, so that 'console is undefined' errors will go away and you won't have to ask if console exists everytime. With this, you just call console.log or any console method anywhere, without problems.

Hope this helps. Cheers

Answer 2

If console itself doesn't exist at all, it throws an error because you're accessing an undefined variable. Just like if(abc) {} throws an error.

Since console resides in window, and window does always exist, this should work:

if(window.console) ... 

Basically, accessing an property that doesn't exist is free and doesn't throw an error (it just evaluates to undefined, failing the if condition). However, it is illegal to access an undeclared variable.