how could `if (console == undefined)` break js execution?

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(){开发者_JAVA技巧}
    };
};

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?

---

(console == undefined)

will throw a ReferenceError if console is undefined.

Use typeof instead which does not throw ReferenceErrors for undeclared variables

(typeof console === "undefined")

---

Try this:

if ( window.console === undefined ) {

    window.console = {
        // Your methods
    }

}

---

It's not really good to compare a variable to undefined, since undefined is a simple undefined variable.

You can use something like this:

if (!window.console)
  console = {
    log   : function(){},
    info  : function(){},
    warn  : function(){},
    error : function(){}
  };

Edit: I just found this: How do I print debug messages in the Google Chrome JavaScript Console?

---

If console is not defined in the js then it will throw an error instead use typeof to check the variable's existense

if (typeof console == 'undefined') { }

---

If you change your if(console == undefined) to if(window.console == undefined) everything will work

---

Any of the answer above should work fine.

One note: You might also add "dir" to your list of empty methods, since people sometimes use console.dir() to print out objects.