Question

When I edit production code (little fixes) I want to add console.log for example, but not to break page for users who don’t have firebug or don’t use chrome I decide to redefine console object:

if (console == undefined) {
    console = {
        log     : function(){},
        info    : function(){},
        warn    : function(){},
        error   : function(){}
    };
};

After I inserted this code, JS-execution was broken in browsers that don’t have console object (IE, firefox without firebug, etc). (By “broken” I mean that code after these lines doesn’t execute at all)
Why did it happen?

Answer 1

(console == undefined)

will throw a ReferenceError if console is undefined.

Use typeof instead which does not throw ReferenceErrors for undeclared variables

(typeof console === "undefined")